The sensitivity of .alpha.-adrenoceptors to phenylephrine as indicated by the ED50 of the rabbit bladder and urethra was compared for segments in vitro from control (estrous), ovariectomized, ovariectomized-estrogen-treated and pregnant animals. In control animals there was a trend for urethral segments to be more sensitive to phenylephrine (lower ED50) than bladder segments. This difference was abolished by castration, which decreased the sensitivity of the anular urethral segment, and by pregnancy, which tended to enhance bladder sensitivity. The tension generated by bladder segments was enhanced by pregnancy and tended to be decreased by castration. The effects of castration tended to be reversed by treatment with estrogen (1 .mu.g/kg of body wt per day for 14 days). Estrogen can influence the response of the urethra and urinary bladder to .alpha.-adrenergic stimulation.