On Metro Bus: You can bring your bike on metro buses (including the Circulator) any time of day seven days a week. There is a rack you can squeeze the lever on and pull down on the front of the bus and you place your front wheel so that the device that you pull up on the side catches over it. This is great for uphill travel and biking in bad weather. I know I take advantage of this often, especially during rush hours when you are not allowed to bring your bike on the metro. You can always use the metro trip planner to figure out the best buses to take.
On Metro Rail: Bikes are allowed on metro during non-rush hours only. Bikes are prohibited during rush hour Monday - Friday from 7:00am - 10:00am and 4:00pm - 7:00pm. Bicycles are also prohibited on Metro during holidays apparently (though I did not realize that until now) and also when the Metro may be busy because of other special events. I imagine this is due to higher traffic flow and safety issues.
Metro allows only two bicycles per train car and you should always enter the train through the side doors and not the center doors. There are three doors on each Metro train car and people with bikes are asked to stay near the ends of the train. Please be sure that your bike must be free of excess dirt & grease and always use the elevator when you have your bike with you.
Metro Questionnaire on City Bikes Site
DC Metro is exploring ways to improve access to the Metro system by foot and by bicycle. We know many of you are conscious of your impact on the environment, and often seek out ways to get from A to B without driving, so be sure to fill out the Questionnaire.