Previous research has shown that benzo-diazepines and barbiturates can increase the proportion of response bursts emitted by rats on differential reinforcement of low rate (DRL) schedules. A recent report found that caffeine could exert a similar effect in mice and the present study was carried out to compare the actions of caffeine with those of chlordiazepoxide on DRL responding in rats and Mongolian gerbils. Chlordiazepoxide increased response rates, decreased reinforcement frequencies and increased percentages of response bursts in both species. Caffeine also increased response rates and decreased reinforcement frequencies, but in neither species was the percentage of response bursts systematically increased. d-Amphetamine administered to the gerbils also did not increase the percentages of response bursts. Combinations of caffeine and chlordiazepoxide were administered to the rats and produced effects which were generally additive.