Abstract Synthetic substance P (SP) was bioassayed in mice by a procedure comprising eleven subtests. Two replications of a dose-response study were conducted at the time of the peak effect of the intravenous injection of SP. Single doses of 31 and 63 ng/kg significantly decreased body temperature. SP, [D-Arg 1]-SP, and [des-NH 2-Arg 1]-SP comparably lowered blood pressure, but [D-Arg 1]-SP and [des-NH 2-Arg 1]-SP were 1 20 to 1 10 as active as SP in lowering body temperature. The activities that lower body temperature and blood pressure may be different. The thyrotropin and luteinizing hormone releasing hormones (TRH and LH-RH) did not lower body temperature. SP also decreased the aggressive response (ED 50, 89 ng/kg).