In three experiments with adult rats, caffeine (0, 10, 20, 40 mg/kg) was found to increase the social investigation of a novel juvenile conspecific. Activity was elevated only after caffeine injections, whereas the duration of social investigation was raised after both the injection of caffeine and the administration of caffeine via the drinking water, suggesting the mechanisms of action of caffeine on these two behaviors may be independent. Duration and frequency of investigation were each increased following caffeine exposure, but the amount of investigation/episode was not influenced by this substance. Finally, the acute response to caffeine was not altered by prior exposure to the methylxanthine in the subject's drinking water.