1. Conscious rats received a saline infusion (5.8 ml . hr-1) via a tail vein, for a 6 hr period. For the final 2 hr of this infusion period, prolactin (7.1 microgram (100 g body wt.)-1 . hr-1) was incorporated in the infusate. A control group of animals continued to receive saline alone. 2. In the first hour of prolactin administration (in comparison with the control group), urine flow, sodium output and osmolal output were all significantly reduced (P less than 0.02); there was also a reduction (P less than 0.05) in renal plasma flow (p-amino-hippurate clearance), but the filtration fraction did not alter. The changes in urinary excretion, and in renal haemodynamics, did not persist for the second hour of prolactin administration, but there was a sustained increase in body fluid volume. 3. It is possible that altered renal haemodynamics are responsible for the reduced urinary water and solute excretion which occurs during acute prolactin administration.