The postnatal development of the 5-HT 1 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: K m = 1 nM and K m = 0.5 μM; these low- and high-affinity constants presumably correspond to 5-HT 1A and 5-HT 1non-A,non-B,non-C (5-HT 1D) respectively. These two activities developed parallelly from the 14–15th to the 28th day. The 8-hydroxy-2-(di- n-propylamino-tetralin) (8-OHDPAT)-induced activity described a curve similar to the one that corresponded to 10 μM 5-HT. These results establish that 5-HT 1A and 5-HT 1non-A,non-B,non-C receptors mainly develop during the synaptogenesis.