When should I exhale and inhale when doing pushups?

Common anaerobic practice says that you should inhale on the way down and exhale on the way up. When doing any kind of weight lifting, the exhale should come during the execution of the exercise, and the inhale should be during the relaxation. This will keep you from holding your breath under sometimes large amounts of pressure built up in your body. It will help prevent injury from hernia, stroke and a number of other issues. :)