Certain neurosteroids rapidly alter the excitability of neurons, in part by potentiating gamma-aminobutyric acid (GABA)-evoked chloride currents, and, like other GABA potentiating drugs, may have anticonvulsant activity. We compared the abilities of a series of isomeric metabolites of progesterone and deoxycorticosterone (3-hydroxy pregnane-20-ones and 3-hydroxy pregnane-21-ol-20-ones) to enhance GABA-evoked chloride currents in cultured hippocampal neurons with their abilities to protect against pentylenetetrazol (PTZ)-induced seizures in mice. Metabolites with 3-hydroxy in the alpha-position and 5-H in the alpha- or beta-configuration were highly effective at potentiating GABA-evoked chloride current and also showed potent anticonvulsant activity in the PTZ seizure test. The corresponding metabolites with hydroxyl groups in the 3 beta-position were considerably less potent in enhancing GABA responses and were inactive in the PTZ test. All of the neurosteroids failed to protect against tonic hindlimb extension in the maximal electroshock seizure test. 5 alpha-Pregnane-3 alpha,11 beta,21-triol-20-one, a corticosterone metabolite reported to block voltage-dependent Ca++ channels, was inactive in either of the anticonvulsant tests. At higher doses, neurosteroids effective in the PTZ test also produced motor impairment. Relative motor toxicity was lower (higher protective index) for compounds with the 5 alpha-configuration than for their corresponding 5 beta-epimers. The anticonvulsant profile of the neurosteroids resembled that of the benzodiazepine clonazepam. Although the anticonvulsant steroids had greater in vitro potencies than clonazepam, they were less potent in vivo, and they had lower protective indices.(ABSTRACT TRUNCATED AT 250 WORDS)