The present study was conducted to determine the effects of skeletal muscle relaxants on contractions of bladder smooth muscle. Rat, rabbit and human bladder muscle strips were incubated in a 50 ml. organ bath filled with physiological salt solution, aerated with 95 per cent O2, 5 per cent CO2 and maintained at 37C. Contractions were induced both electrically and with acetylcholine. Contractile strength was measured before and after 30 to 70 minute incubations with diazepam (0.25 to 1.0 microgram./ml.), dantrolene sodium (4 micrograms./ml.) or baclofen (4 micrograms./ml.). Neither baclofen nor dantrolene sodium had any effect (p greater than 0.3) on electrically or acetylcholine-induced contractions in any of the 3 species tested but diazepam potentiated both electrically induced (115 per cent of predrug contractile strength, p less than 0.05) and acetylcholine-induced (122 per cent of predrug contractile strength, p less than 0.01) contractions of rat, but not rabbit or human, bladder muscle. In rabbits diazepam significantly reduced maximal bladder pressures in vivo (p less than 0.05) but had no effect on bladder capacity. Dantrolene sodium had no effect on rabbit bladder pressure or capacity in vivo. While diazepam is thought to exert its effects on skeletal muscle through CNS inhibition, these results suggest that diazepam may affect rat but not rabbit or human bladder smooth muscle directly.