Oxytocin (OT) and arginine vasopressin (AVP) have been found to be present in the reproductive system of the mammalian male, but their function in this system is unclear. This study examined the effects of these two hormones on contractions of the rat caput epididymis, as well as the hormones' effects on epididymal diameter during contractions. The initial segments of caput epididymides were observed in vitro in a solution of modified Tyrode's solution, with a test solution containing a concentration of either OT or AVP being added after a 5-min period. The frequency and diameter of the epididymal contractions were measured before and after addition of each test solution. Also observed was the effect of magnesium concentrations on the ability of OT to induce changes in epididymal contractility. This study found that the frequency of epididymal contractions increase and that the diameter of the epididymal tubules decrease proportionally during contractions with the addition of either OT or AVP. Furthermore, we noted that Mg++ has an inverse correlation with the frequency of contractions of the caput epididymis both in the presence and absence of OT. These findings suggest that both OT and AVP may have a role in regulating epididymal contractility and thus, perhaps, sperm transport through that organ.