The possibility that ADH [antidiuretic hormone] was present in an ovine prolactin preparation (NIH [National Institute of Health]-P-S10) in quantities sufficient to cause antidiuresis was examined in male rats. Compared with saline-injected controls, injection of 20 .mu.g prolactin significantly decreased (P < 0.001) urine flow and significantly increased (P < 0.006) urine osmolality 15 min after injection. The time course of the changes in both flow and osmolality and the direction of the changes were similar to those which can be produced by an i.v. injection of ADH. When 40 .mu.u [microunits] of ADH dissolved in 0.2 ml saline were injected into rats subjected to the same protocol as those given prolactin, the changes in urine flow and urine osmolality 15 min after injection were not statistically different (P < 0.5 and P < 0.2, respectively) than corresponding changes in the prolactin treated rats. The antidiuresis and increase in urine osmolality produced by the i.v. injection of ovine prolactin in rats could be accounted for by trace amounts of ADH contaminating the ovine prolactin sample.