Caerulein, cholecystokinin octapeptide (CCK-8) and diazepam delayed the onset of seizures produced by harman and thiosemicarbazide (TSC). Caerulein had the potency of diazepam, whereas CCK-8 was less active by a factor of four. The convulsions induced by isoniazid (INH) were very resistant to both caerulein and diazepam; CCK-8 was not tested against isoniazid. Haloperidol did not influence the effect of TSC; it enhanced isoniazid-induced seizures, and antagonized the convulsant effect of harman.