The possibility that 5-hydroxytryptamine (5-HT) stimulates testicular maturation in the red swamp crayfish, Procambarus clarkii, was investigated. A series of 5-HT agonists and antagonists was used in addition to 5-HT itself. 5-HT and its agonists not only induced testicular maturation but also development of the androgenic glands. In contrast, the 5-HT antagonists had no stimulatory effects on the testes or androgenic gland. This stimulatory action of 5-HT is hypothesized to be indirect, 5-HT stimulating release of the gonad-stimulating hormone, which in turn activates the androgenic gland to synthesize and release androgenic gland hormone, and the androgenic gland hormone then triggers testicular maturation.