1. The effect of infusion of ovine prolactin was studied in anaesthetized dogs pretreated with bromocryptine to reduce the release of endogenous prolactin. 2. Prolactin, injected intravenously and also directly into one kidney, resulted in a 12–18% increase in glomerular filtration rate (GFR) by both kidneys. 3. This increased GFR was not associated with any demonstrable changes in whole-kidney blood flow, distribution of intrarenal blood flow, fractional excretion of sodium or osmolar or free-water clearance. 4. We conclude that ovine prolactin produced an increase in GFR not dependent on an increase in whole-kidney plasma flow.