Nicotine addiction is a serious health problem. Almost 35 million people in the United States try to quit smoking each year, but less than 7 percent are successful. Many smokers report a "high" sensation, followed by a feeling of relaxation. Such positive reinforcement factors, combined with nicotine cravings, make it difficult to stop the use of nicotine products. Past research has suggested that women may experience fewer nicotine cravings and may be less affected by smoking's positive reinforcement factors when compared to men. Fluctuations in levels of the female hormones estradiol and progesterone during menstruation may play a role in determining how women are affected by nicotine. The purpose of this study is to examine the effects of a controlled oral dose of progesterone on smoking behavior, nicotine withdrawal symptoms, and nicotine-related positive reinforcement factors in nicotine dependent males and females.