Graded doses of nicotine (0.01-1.0 mg/kg) were administered alone and together with three doses of diazepam (0.3, 1.0 and 3.0 mg/kg) to rats responding on a fixed-interval 2-min schedule of liquid food reinforcement. Nicotine (0.03-1.0 mg/kg) increased overall rate, but diazepam had little effect. Both nicotine and the two highest doses of diazepam attenuated the change in response rate through the interval. When combined with nicotine the lowest dose of diazepam increased overall rates above those produced by nicotine alone. However, it appeared to diminish the effects of nicotine on the within-interval pattern of responding. These changes appeared to be due to an elevation in the high rates at the end of the interval. In contrast, the highest diazepam dose increased overall response rates when combined with low doses of nicotine, but decreased the high rates observed after larger nicotine doses. This dose of diazepam combined in an additive manner with nicotine to reduce the degree of response rate change within the interval. The interaction between nicotine and diazepam depends on the aspect of behavior under investigation and the particular doses of the two drugs.