The Genetically Epilepsy-Prone Rat (GEPR) is a widely studied model of epileptiform disorders. While there is considerable evidence that neurotransmitter abnormalities contribute to the unusual sensitivity of these animals to seizures, the possibility that seizure susceptibility may reflect developmental changes in the central nervous system has not been fully addressed. In the present study, 91 GEPR-9 pups were tested for incidence, latency and severity of an audiogenically induced seizure at 6, 9, 12, 15, 18, 21, 24, 27, 28, 29, or 30 days postpartum (1 test per pup) and retested at 60 days. Seizure incidence, latency, and severity were significantly greater on Days 27, 28, 29, and 30 than on all previous days. The first observation of running fits occurred in Day 18 pups and the first evidence of seizures occurred in Day 21 pups. When retested at Day 60, seizure incidence and severity were significantly greater than on initial tests while latency declined. The results suggest that seizure susceptibility in the GEPR-9 occurs as the result of developmental events in the CNS occurring on or shortly after Day 18 postpartum.