The serum half-life of feline intestinal alkaline phosphatase (ALP) in the cat was 2 minutes, and that of feline hepatic ALP, 5.8 hours. The feline hepatic ALP could not be identified in the urine of the cats, nor was the clearance rate affected by bilateral nephrectomy. These data indicated the shor serum half-life of hepatic ALP was not a reusult of renal excretion. Since canine hepatic ALP cleared from feline blood at the same rate as the feline hepatic ALP, possibly the discrepancy in hepatic ALP half-life between the species was related to the cats' ability to clear the blood of the enzyme.