 Participants must be healthy, between 18 and 55 years of age, and report a cigarette intake of 15 cigday for at least 1 year. They must provide an afternoon screening CO level of 15 ppm at intake. They must also provide a urine sample for cotinine analysis, and receive a result of at least 4 on the urine immunoassay test strip scale possible range 0-6 at intake. 