diff --git a/src/nix/local.mk b/src/nix/local.mk
index b057b7cc6..8ed8eaef7 100644
--- a/src/nix/local.mk
+++ b/src/nix/local.mk
@@ -15,11 +15,12 @@ nix_SOURCES := \
   $(wildcard src/nix-prefetch-url/*.cc) \
   $(wildcard src/nix-store/*.cc) \
 
-nix_CXXFLAGS += -I src/libutil -I src/libstore -I src/libfetchers -I src/libexpr -I src/libmain
+# -fpermissive is needed by lowdown.
+nix_CXXFLAGS += -I src/libutil -I src/libstore -I src/libfetchers -I src/libexpr -I src/libmain -fpermissive
 
 nix_LIBS = libexpr libmain libfetchers libstore libutil
 
-nix_LDFLAGS = -pthread $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) -lboost_context -lboost_thread -lboost_system
+nix_LDFLAGS = -pthread $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) -lboost_context -lboost_thread -lboost_system -llowdown
 
 $(foreach name, \
   nix-build nix-channel nix-collect-garbage nix-copy-closure nix-daemon nix-env nix-hash nix-instantiate nix-prefetch-url nix-shell nix-store, \
diff --git a/src/nix/markdown.cc b/src/nix/markdown.cc
new file mode 100644
index 000000000..40788a42f
--- /dev/null
+++ b/src/nix/markdown.cc
@@ -0,0 +1,50 @@
+#include "markdown.hh"
+#include "util.hh"
+#include "finally.hh"
+
+#include <sys/queue.h>
+extern "C" {
+#include <lowdown.h>
+}
+
+namespace nix {
+
+std::string renderMarkdownToTerminal(std::string_view markdown)
+{
+    struct lowdown_opts opts {
+        .type = LOWDOWN_TERM,
+        .maxdepth = 20,
+        .cols = std::min(getWindowSize().second, (unsigned short) 80),
+        .hmargin = 0,
+        .vmargin = 0,
+        .feat = LOWDOWN_COMMONMARK | LOWDOWN_FENCED | LOWDOWN_DEFLIST | LOWDOWN_TABLES,
+        .oflags = 0,
+    };
+
+    auto doc = lowdown_doc_new(&opts);
+    if (!doc)
+        throw Error("cannot allocate Markdown document");
+    Finally freeDoc([&]() { lowdown_doc_free(doc); });
+
+    size_t maxn = 0;
+    auto node = lowdown_doc_parse(doc, &maxn, markdown.data(), markdown.size());
+    if (!node)
+        throw Error("cannot parse Markdown document");
+    Finally freeNode([&]() { lowdown_node_free(node); });
+
+    auto renderer = lowdown_term_new(&opts);
+    if (!renderer)
+        throw Error("cannot allocate Markdown renderer");
+    Finally freeRenderer([&]() { lowdown_term_free(renderer); });
+
+    auto buf = lowdown_buf_new(16384);
+    if (!buf)
+        throw Error("cannot allocate Markdown output buffer");
+    Finally freeBuffer([&]() { lowdown_buf_free(buf); });
+
+    lowdown_term_rndr(buf, nullptr, renderer, node);
+
+    return std::string(buf->data, buf->size);
+}
+
+}
diff --git a/src/nix/markdown.hh b/src/nix/markdown.hh
new file mode 100644
index 000000000..78320fcf5
--- /dev/null
+++ b/src/nix/markdown.hh
@@ -0,0 +1,7 @@
+#include "types.hh"
+
+namespace nix {
+
+std::string renderMarkdownToTerminal(std::string_view markdown);
+
+}
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index d370ca767..c7f8af742 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -32,6 +32,7 @@ extern "C" {
 #include "globals.hh"
 #include "command.hh"
 #include "finally.hh"
+#include "markdown.hh"
 
 #if HAVE_BOEHMGC
 #define GC_INCLUDE_NEW
@@ -518,10 +519,16 @@ bool NixRepl::processLine(string line)
             while (v2->type == tPrimOpApp)
                 v2 = v2->primOpApp.left;
             if (v2->primOp->doc) {
-                // FIXME: format markdown.
-                if (!v2->primOp->args.empty())
-                    std::cout << fmt("Arguments: %s\n\n", concatStringsSep(" ", v2->primOp->args));
-                std::cout << trim(stripIndentation(v2->primOp->doc)) << "\n";
+                auto args = v2->primOp->args;
+                for (auto & arg : args)
+                    arg = "*" + arg + "*";
+
+                auto markdown =
+                    "**Synopsis:** `builtins." + (std::string) v2->primOp->name + "` "
+                    + concatStringsSep(" ", args) + "\n\n"
+                    + trim(stripIndentation(v2->primOp->doc));
+
+                std::cout << renderMarkdownToTerminal(markdown);
             } else
                 throw Error("builtin function '%s' does not have documentation", v2->primOp->name);
         } else