Renal function was studied in anesthetized starlings by micropuncture and clearance techniques. The effects of intravenous infusions of isosmotic sodium chloride and hyposmotic mannitol solutions were compared. In the proximal tubules of the superficial reptilian-type nephrons, there is net reabsorption of sodium, chloride, and potassium; net secretion of calcium and urate; and either net secretion or no net secretion or reabsorption of magnesium and phosphate. The fractional delivery of ions and urate to the site of micropuncture, the percent of fluid reabsorbed, and the single nephron glomerular filtration rate (SNGFR) were not significantly different between the two infusion treatments. However, in the sodium chloride-infused birds, whole kidney GFR and the fractional excretion of sodium and chloride by the whole animal were higher, and fractional excretion of magnesium and potassium were lower. The data for each infusion group were further subdivided on the basis of whether the birds were in molt. In starlings, molt is associated with elevated levels of plasma prolactin. For the sodium chloride infusion, sodium and water reabsorption to the site of micropuncture was greater in the birds in molt than in those not in molt.