The in vivo concentration-anticonvulsant effect relationships of six benzodiazepines, midazolam, clonazepam, oxazepam, flunitrazepam, diazepam and clobazam were quantified in individual rats and correlated with the affinity to the GABAA-benzodiazepine receptor complex. Furthermore the interaction between midazolam and the benzodiazepine antagonist flumazenil was characterized. All benzodiazepines exhibited a nonlinear relationship between concentration and anticonvulsant effect without ceiling of the effect at higher concentration. The potency of most benzodiazepines was similar with values of the EC250, between 0.067 +/- 0.01 mg. l-1 for midazolam and 0.21 +/- 0.03 mg. l-1 for diazepam. The EC250,u of clobazam was 2.8 +/- 0.9 mg. l-1. These values were considerably larger than the Ki for binding at the GABAA-benzodiazepine receptor complex. No correlation was observed between EC250,u and Ki. Antagonism of the anticonvulsant effect of midazolam by flumazenil was associated with a remarkable change in the concentration-anticonvulsant effect relationship. Analysis of these data on basis of a composite model provided evidence for two separate effects of which only one is antagonized by flumazenil. The anticonvulsant effect at low midazolam concentration was characterized on basis of the sigmoid E maximal effect pharmacodynamic model. The value of the EC50,u was 0.0086 +/- 0.0013 mg. l-1 which is similar to the Ki for binding at the GABAA-benzodiazepine receptor complex. The second more pronounced anticonvulsant effect occurred at higher concentration and was described by an exponential function. The findings of this study indicate that the effect of benzodiazepines against seizures induced by cortical stimulation in vivo cannot be fully accounted for by an interaction at the GABAA-benzodiazepine receptor complex.