diff --git a/src/extension/infer.rs b/src/extension/infer.rs
index 9024c66c96..5b5d260550 100644
--- a/src/extension/infer.rs
+++ b/src/extension/infer.rs
@@ -857,21 +857,28 @@ mod test {
         let [w] = mult.outputs_arr();
 
         builder.set_outputs([w])?;
-        let hugr = builder.base;
-        // TODO: when we put new extensions onto the graph after inference, we
-        // can call `finish_hugr` and just look at the graph
-        let (solution, extra) = infer_extensions(&hugr)?;
-        assert!(extra.is_empty());
+        let mut hugr = builder.base;
+        let closure = hugr.infer_extensions()?;
+        assert!(closure.is_empty());
         assert_eq!(
-            *solution.get(&(src.node(), Direction::Outgoing)).unwrap(),
+            hugr.get_nodetype(src.node())
+                .signature()
+                .unwrap()
+                .output_extensions(),
             rs
         );
         assert_eq!(
-            *solution.get(&(mult.node(), Direction::Incoming)).unwrap(),
+            hugr.get_nodetype(mult.node())
+                .signature()
+                .unwrap()
+                .input_extensions,
             rs
         );
         assert_eq!(
-            *solution.get(&(mult.node(), Direction::Outgoing)).unwrap(),
+            hugr.get_nodetype(mult.node())
+                .signature()
+                .unwrap()
+                .output_extensions(),
             rs
         );
         Ok(())
diff --git a/src/hugr.rs b/src/hugr.rs
index 4e1c52edf5..439888784d 100644
--- a/src/hugr.rs
+++ b/src/hugr.rs
@@ -193,7 +193,7 @@ impl Hugr {
         rw.apply(self)
     }
 
-    /// Infer extension requirements
+    /// Infer extension requirements and add new information to `op_types` field
     pub fn infer_extensions(
         &mut self,
     ) -> Result<HashMap<(Node, Direction), ExtensionSet>, InferExtensionError> {
@@ -202,9 +202,22 @@ impl Hugr {
         Ok(extension_closure)
     }
 
-    /// TODO: Write this
-    fn instantiate_extensions(&mut self, _solution: ExtensionSolution) {
-        //todo!()
+    /// Add extension requirement information to the hugr in place.
+    fn instantiate_extensions(&mut self, solution: ExtensionSolution) {
+        // We only care about inferred _input_ extensions, because `NodeType`
+        // uses those to infer the output extensions
+        for ((node, _), input_extensions) in solution
+            .iter()
+            .filter(|((_, dir), _)| *dir == Direction::Incoming)
+        {
+            let nodetype = self.op_types.try_get_mut(node.index).unwrap();
+            match nodetype.signature() {
+                None => nodetype.input_extensions = Some(input_extensions.clone()),
+                Some(existing_ext_reqs) => {
+                    debug_assert_eq!(existing_ext_reqs.input_extensions, *input_extensions)
+                }
+            }
+        }
     }
 }
 
@@ -366,7 +379,14 @@ impl From<HugrError> for PyErr {
 
 #[cfg(test)]
 mod test {
-    use super::Hugr;
+    use super::{Hugr, HugrView, NodeType};
+    use crate::extension::ExtensionSet;
+    use crate::hugr::hugrmut::HugrInternalsMut;
+    use crate::ops;
+    use crate::type_row;
+    use crate::types::{FunctionType, Type};
+
+    use std::error::Error;
 
     #[test]
     fn impls_send_and_sync() {
@@ -385,4 +405,55 @@ mod test {
         let hugr = simple_dfg_hugr();
         assert_matches!(hugr.get_io(hugr.root()), Some(_));
     }
+
+    #[test]
+    fn extension_instantiation() -> Result<(), Box<dyn Error>> {
+        const BIT: Type = crate::extension::prelude::USIZE_T;
+        let r = ExtensionSet::singleton(&"R".into());
+
+        let root = NodeType::pure(ops::DFG {
+            signature: FunctionType::new(type_row![BIT], type_row![BIT]).with_extension_delta(&r),
+        });
+        let mut hugr = Hugr::new(root);
+        let input = hugr.add_node_with_parent(
+            hugr.root(),
+            NodeType::pure(ops::Input {
+                types: type_row![BIT],
+            }),
+        )?;
+        let output = hugr.add_node_with_parent(
+            hugr.root(),
+            NodeType::open_extensions(ops::Output {
+                types: type_row![BIT],
+            }),
+        )?;
+        let lift = hugr.add_node_with_parent(
+            hugr.root(),
+            NodeType::open_extensions(ops::LeafOp::Lift {
+                type_row: type_row![BIT],
+                new_extension: "R".into(),
+            }),
+        )?;
+        hugr.connect(input, 0, lift, 0)?;
+        hugr.connect(lift, 0, output, 0)?;
+        hugr.infer_extensions()?;
+
+        assert_eq!(
+            hugr.op_types
+                .get(lift.index)
+                .signature()
+                .unwrap()
+                .input_extensions,
+            ExtensionSet::new()
+        );
+        assert_eq!(
+            hugr.op_types
+                .get(output.index)
+                .signature()
+                .unwrap()
+                .input_extensions,
+            r
+        );
+        Ok(())
+    }
 }