The BZ1 (omega 1)-selective compound, zolpidem, is a clinically effective hypnotic drug with a pharmacological profile which differs from those of benzodiazepine anxiolytics and hypnotics. Zaleplon (CL 284,846) has recently been described as a hypnotic agent which also has BZ1 (omega 1) receptor selectivity. The pharmacological effects of zolpidem and zaleplon were therefore compared in mice and rats. Both drugs blocked tonic convulsions induced in mice by pentylenetetrazole and electroconvulsive shock and clonic convulsions induced by isoniazid. Zaleplon was more potent than zolpidem but the maximal effect of zolpidem for increasing the latency to isoniazid-induced convulsions was greater than that of zaleplon. Little tolerance developed to the anticonvulsant effect of zaleplon against isoniazid-induced seizures following twice daily administration of 10 or 30 mg/kg for 10 days. Both compounds reduced locomotor activity and produced motor deficits in the rotarod and loaded grid tests in mice. However, while zaleplon produced all three effects at similar doses, zolpidem showed the greatest potency for reducing locomotion. Zaleplon and zolpidem also decreased locomotion and produced a rotarod deficit in rats. Again, the difference between the doses giving rise to these two effects was greater for zolpidem than for zaleplon. In a drug discrimination procedure using rats trained to discriminate a dose (5 mg/kg) of chlordiazepoxide, zaleplon produced partial substitution for chlordiazepoxide at doses which greatly reduced response rates. These results show that zaleplon and zolpidem have similar pharmacological profiles, presumably related to their BZ1 (omega 1) receptor selectivity. However, the difference between doses producing motor deficits (rotarod, loaded grid) and those giving rise to other effects (anticonvulsant, decreased locomotion) was greater for zolpidem than for zaleplon. This difference may be related to a greater in vivo intrinsic activity of zolpidem as indicated by the different efficacies of the two drugs to antagonise isoniazid-induced convulsions.