We investigated the possibility that PGE1, at approximately the minimal seminal plasma concentration reported to be associated with human fertility (3), placed in the vaginas of female dogs might produce local vasodilation and thereby increase vaginal blood flow. Fifteen micrograms of PGE1 in 1 ml, 38 degrees C saline increased vaginal blood flow 2.5 times (0.27 +/- 0.10 to 0.67 +/- 0.10 ml/g X min) as compared to control (1 ml, 38 degrees C saline, intravaginally) without altering arterial pressure, hepatic blood flow, or renal blood flow. We conclude that seminal fluid PGE1 can produce vaginal vasodilation. The role of seminal fluid PG induced increases in vaginal blood flow upon reproductive success, if any, is entirely speculative.