We have examined the effects of pentylenetetrazol-induced epileptic seizures on brain-derived neurotrophic factor messenger RNA and protein and on the messenger RNA of its receptor in the rat. Pentylenetrazol, which acts at the picrotoxin recognition site of the GABAA receptor, was injected intraperitoneally and induced seizures by decreasing the inhibitory GABAergic activity. The effects of a single acute convulsive dose (50 mg/kg) of pentylenetetrazol were analysed at different time points by in situ hybridization or immunohistochemistry. Kindling was induced by daily subconvulsive injections (30 mg/kg) of pentylenetetrazol. At different time points during the kindling process, the messenger RNAs of brain-derived neurotrophic factor and trkB and the protein levels of brain-derived neurotrophic factor were analysed. We showed that brain-derived neurotrophic factor messenger RNA dramatically increased in neurons of the granule cell layer, piriform cortex and amygdala 3 h but not 6 h after an acute high dose of pentylenetetrazol, while brain-derived neurotrophic factor-like immunoreactivity was decreased in the granule cell layer and neurons of the hilus. The trkB messenger RNA was similarly increased 3 h and 6 h after the injection and returned to control levels after 24 h. The first change during the kindling development was seen after the first severe seizure: brain-derived neurotrophic factor messenger RNA was markedly increased in the piriform cortex and amygdala but not in the hippocampus. In fully kindled rats, which had several severe seizures, brain-derived neurotrophic factor messenger RNA and trkB messenger RNA were unaffected 3 h and 24 h after the last pentylenetetrazol injection. However, brain-derived neurotrophic factor-like immunoreactivity was markedly increased in the hippocampal formation 3 h, 24 h and three days after the last pentylenetetrazol injection, and still increased after 10 days. These results suggest that brain-derived neurotrophic factor may be involved in protection mechanisms after damage during seizures and in sprouting responses. The piriform cortex/amygdala seems to be an area of origin for the kindling development.