The anticonvulsant actions of lamotrigine and phenytoin against pentylenetetrazol-induced seizures were compared in laboratory rats during ontogenesis. Both drugs (lamotrigine in doses of 2.5, 5, 10, and 20 mg/kg and phenytoin in doses of 5, 10, 30 and 60 mg/kg) were injected intraperitoneally 30 min before pentylenetetrazol. Phenytoin and lamotrigine did not affect the incidence or latency of minimal (i.e., predominantly clonic, nongeneralized) seizures, although pretreatment with phenytoin changed the pattern of this phenomenon from short (10-30-s) seizures to long-lasting 'status of minimal seizures'. Both drugs abolished selectively the tonic phase of major, i.e., generalized tonic-clonic seizures, usually without any influence on the clonic phase of these seizures. Only the highest dose of phenytoin in adult animals suppressed the generalized tonic-clonic seizures as a whole. The study did not reveal any change of action of lamotrigine or phenytoin against pentylenetetrazol-induced motor seizures throughout development.