Whole kidney and renal micropuncture techniques were used to investigate the effects of chronic prolactin treatment on kidney function in anaesthetized female rats. At the whole kidney level, glomerular filtration rate (GFR) and fluid reabsorption were both significantly (P<0·02) increased in the hormone-treated group. At the single nephron level, GFR and proximal fluid reabsorption were also increased (P<0·05) by prolactin treatment. Fractional reabsorption was also enhanced at the proximal tubular level in hormone-treated animals. Such changes in renal function are similar to those seen in rat pregnancy and cervically stimulated pseudopregnancy. Since circulating prolactin concentrations are increased in both reproductive states, the hormone may play an important role in establishing the characteristic renal changes seen therein.