In experiments on male albino rats and mice a study is made of the effects of imidazol which is a phosphodiesterase stimulator, papaverine which inhibits phosphodiesterase and histamine which stimulates adenylate cyclase, on the convulsive-seizure reactivity. The substances are introduced intraventricularly and intracerebroventricularly, imidazol also intraperitoneally in different doses and at different intervals before the convulsive agent. Electrical, pentylenetetrazol (Cor) and strychnine convulsion models are used. The effect of imidazol on the spontaneous cortical bioelectrical activity is studied throuth its i. v. administration in rabbits. Imidazol markedly increases the convulsive reactivity, and in large doses it alone results in electrographic and motor convulsions. Paperine slightly lowers the convulsive-seizure reactivity only in pentylenetetrazol convulsions. The results obtained and their comparison with the results of previous experiments of ours with other drugs affecting the cyclic adenosinemonophosphate (cAMP) system, such as lithium, haloperidol, caffeine and theophyline, do not permit to assume a considerable significance of the influence of these substances (in the doses tested) on the cAMP system in the mechanisms of their effects on the convulsive-seizure reactivity.