The postnatal development of the 5-HT1 receptor system was studied in young rat brain cortex from birth to adulthood (14 successive ages). The high-affinity binding of [3H]5-HT was low at birth but developed markedly between the 8th and the 15th day postnatally. The basal adenylate cyclase activity produced 50 pmoles cAMP/mg protein/min at birth and increased from the 8th to the 15th day. 5-HT could stimulate the adenylate cyclase activity in adult rat brain cortex with two different affinity constants: Km = 1 nM and Km = 0.5 microM; these low- and high-affinity constants presumably correspond to 5-HT1A and 5-HT1non-A.non-B.non-C (5-HT1D) respectively. These two activities developed parallelly from the 14-15th to the 28th day. The 8-hydroxy-2-(di-n-propylamino-tetralin) (8-OH-DPAT)-induced activity described a curve similar to the one that corresponded to 10 microM 5-HT. These results establish that 5-HT1A and 5-HT1non-A.non-B.non-C receptors mainly develop during the synaptogenesis.