diff --git a/setup.py b/setup.py
index <HASH>..<HASH> 100644
--- a/setup.py
+++ b/setup.py
@@ -103,7 +103,7 @@ http://api.mongodb.org/python/current/installation.html#osx
 
 kwargs = {}
 
-version = "4.5.1"
+version = "5.0.dev1"
 
 with open('README.rst') as f:
     kwargs['long_description'] = f.read()
diff --git a/tornado/__init__.py b/tornado/__init__.py
index <HASH>..<HASH> 100644
--- a/tornado/__init__.py
+++ b/tornado/__init__.py
@@ -25,5 +25,5 @@ from __future__ import absolute_import, division, print_function
 # is zero for an official release, positive for a development branch,
 # or negative for a release candidate or beta (after the base version
 # number has been incremented)
-version = "4.5.1"
-version_info = (4, 5, 1, 0)
+version = "5.0.dev1"
+version_info = (5, 0, 0, -100)
Bump master version to <I>.dev1