Acute changes in renal excretion occurred when pen-fed sheep were given their daily feed. There was a reduction in the rate of urine flow and the concentration of the urine increased. During the oliguric phase, sodium and potassium excretion decreased whereas hydrogen ion excretion increased and caused acidification (pH 5–6) of the normally alkaline urine (pH 7–8). Renal clearances of inulin and PAH did not indicate that the urinary changes were due to altered renal haemodynamics. It is suggested that the observed renal response was a reflection of the sudden shift of extracellular fluid into the gut at the onset of feeding.