Effects of induced cholestasis and hepatocellular necrosis and of fasting on serum biochemical constituents including bile acids, IgA, bilirubin, alkaline phosphatase, gamma-glutamyltransferase (GGT), arginase, and the clearance of sodium sulfobromophthalein were studied in 4 groups of equids. The reference value for serum bile acids, as determined by an enzymatic colorimetric procedure for horses and ponies was 5.94 +/- 2.72 mumol/L, there being no statistical difference for horses and ponies. Sample collection at time of feeding had no effect on serum bile acid concentration. Seemingly, serum bile acids, arginase, and GGT were the most sensitive indicators of cholestasis and/or hepatocellular necrosis and would form an essential minimum effective battery of tests to diagnose and prognose hepatic disease in equids. These tests provided a measure of hepatobiliary transport function (bile acids), cell necrosis (arginase), and cholestasis (GGT and bile acids).