SUMMARY1. The renal effects of acute and chronic progesterone and prolactin administration were investigated in rats.2. Acute progesterone treatment caused a reduced urinary potassium excretion compared to controls (0-80 + 0±03 vs. 1-30 + 0 04 molee min-; P < 0 01). No other renal changes were apparent.3. Acute prolactin administration produced no significant changes in renal function.4. Chronic progesterone treatment reduced urinary potassium output by increasing tubular reabsorption. Absolute reabsorption moleoe min-) for controls was 4 70 + 0-42 and for progesterone treatment, 6-40 + 0 57 (P < 0.05 comparing the two).No other renal changes were apparent. 5. Animals made pseudopregnant by chronic prolactin administration showed a significant (16 %) elevation in glomerular filtration rate, similarly enhanced fluid and solute reabsorption, and a significant (16 %) increase in proximal tubule length. This would imply a role for prolactin in early rat pregnancy and pseudopregnancy when similar renal changes are evident and circulating hormone levels high.