Abnormal function of serotonergic neurones may be involved in the age-related susceptibility of DBA/2J mice to generalised convulsions induced by auditory stimulation. We have measured 5-HT2 receptor binding sites and synaptosomal 5-HT uptake in 5 brain regions of DBA/2J mice at ages before, during and after their maximal susceptibility to audiogenic seizures and in age-matched C57 B1/6 mice, a strain resistant to audiogenic seizures at all ages. The number of 5-HT2 binding sites was 20% higher in the cerebral cortex of DBA/2J than C57 B1/6 mice at the time of maximal susceptibility of DBA/2J mice to audiogenic seizures but did not differ at other ages. The number of 5-HT2 binding sites did not differ between the two strains at the ages studied in forebrain, mid-brain, hippocampus and pons-medulla. A marked reduction in the number of 5-HT2 binding sites was apparent in the mid-brain, hippocampus and pons-medulla of both strains of mice between 13-15 days of age and 21-23 days of age. Synaptosomal 5-HT uptake did not differ significantly between DBA/2J and C57 B1/6 in any of the brain regions at the ages studied. The higher density of cortical 5-HT2 binding sites in DBA/2J mice may contribute to their susceptibility to sound-induced seizures.