diff --git a/eval/builtin_func.go b/eval/builtin_func.go
index a558a4c5..88c5a23f 100644
--- a/eval/builtin_func.go
+++ b/eval/builtin_func.go
@@ -62,6 +62,7 @@ func init() {
 		&BuiltinFn{"put-all", wrapFn(putAll)},
 		&BuiltinFn{"unpack", wrapFn(unpack)},
 
+		&BuiltinFn{"to-json", wrapFn(toJSON)},
 		&BuiltinFn{"from-json", wrapFn(fromJSON)},
 
 		&BuiltinFn{"kind-of", kindOf},
@@ -337,6 +338,18 @@ func unpack(ec *EvalCtx) {
 	}
 }
 
+// toJSON converts a stream of Value's to JSON data.
+func toJSON(ec *EvalCtx) {
+	in := ec.ports[0].Chan
+	out := ec.ports[1].File
+
+	enc := json.NewEncoder(out)
+	for v := range in {
+		err := enc.Encode(v)
+		maybeThrow(err)
+	}
+}
+
 // fromJSON parses a stream of JSON data into Value's.
 func fromJSON(ec *EvalCtx) {
 	in := ec.ports[0].File
diff --git a/eval/list.go b/eval/list.go
index 4d650ff6..b75c8812 100644
--- a/eval/list.go
+++ b/eval/list.go
@@ -2,6 +2,7 @@ package eval
 
 import (
 	"bytes"
+	"encoding/json"
 	"errors"
 	"strconv"
 	"strings"
@@ -42,6 +43,10 @@ func (l List) Repr(indent int) string {
 	return b.String()
 }
 
+func (l List) MarshalJSON() ([]byte, error) {
+	return json.Marshal(*l.inner)
+}
+
 func (l List) Len() int {
 	return len(*l.inner)
 }
diff --git a/eval/map.go b/eval/map.go
index ebf9655e..0f76d05b 100644
--- a/eval/map.go
+++ b/eval/map.go
@@ -1,6 +1,9 @@
 package eval
 
-import "errors"
+import (
+	"encoding/json"
+	"errors"
+)
 
 // Map is a map from string to Value.
 type Map struct {
@@ -21,6 +24,15 @@ func (Map) Kind() string {
 	return "map"
 }
 
+func (m Map) MarshalJSON() ([]byte, error) {
+	// XXX Not the most efficient way.
+	mm := map[string]Value{}
+	for k, v := range *m.inner {
+		mm[ToString(k)] = v
+	}
+	return json.Marshal(mm)
+}
+
 func (m Map) Repr(indent int) string {
 	var builder MapReprBuilder
 	builder.Indent = indent