Retrograde ejaculation is partial or total propulsion of semen from the posterior urethra into the urinary bladder; it is well characterized (and relatively common) in humans, with only a few reports in animals. Our objectives were to determine whether retrograde flow of semen occurred during ejaculation in mature dogs with normal fertility, and to determine the effects of phenylpropanolamine on this phenomenon (dose-titration, switch-back study). Retrograde ejaculation and urethral pressure profile measurements were evaluated (double-blind) in six dogs after 5 days of oral treatment with phenylpropanolamine (0, 2, 4, or 8 mg/kg); all dogs received all treatments (at 2-week intervals). The number of sperm in the urine was determined before and after each manual sperm collection. Urethral pressure profiles were obtained three times during each procedure. In the absence of phenylpropanolamine, sperm were present in the bladder after semen collection in all dogs (number varied significantly among individuals). The mean (+/-S.D.) number of sperm in the bladder was 17.0+/-5.0, 18.5+/-1.2, 5.1+/-5.0, and 4.8+/-0.1 x 10(6) sperm for 0, 2, 4, and 8 mg/kg, respectively (no significant difference between dogs given 4 or 8 mg/kg, but both were significantly lower than those given 0 or 2mg/kg). This reduction was significantly correlated to the increase in mean urethral pressure at the level of the sphincter (39 cm versus 59 cm H(2)O in placebo-treated dogs versus those given 8 mg/kg). In conclusion, we confirmed that retrograde ejaculation occurred during the ejaculatory process in normal dogs, and we demonstrated that phenylpropanolamine (4 or 8 mg/kg once daily for 5 days before collection) increased urethral pressure and reduced the number of sperm voided into the bladder during ejaculation.