3H-Flunitrazepam (500 pM) was used to estimate the inhibitory effects of caffeine, theophylline, theobromine and 3-methylxanthine on binding to rat brain homogenates. Caffeine and theophylline showed a low affinity competitive type of inhibition in vitro, but no inhibition in vivo. Low concentrations of caffeine failed to show clear inhibitory capabilities against low concentrations of 3H-flunitrazepam. The results suggest that only the toxic central nervous system effects of methylxanthines are mediated via the benzodiazepine receptor.