Antipyrine (AP) clearance was determined from AP plasma decay in 57 rats at various times after partial hepatectomy. This value for AP clearance correlated closely (r = 0.99) to AP clearance estimated from a single plasma AP concentration measured 5 h after AP administration. An AP apparent volume of distribution was taken as 0.66 liters per kilogram body weight. The error introduced by estimating AP apparent volume of distribution as 0.66 times body weight was negligible, even when only a single plasma sample for AP concentration served to calculated AP clearance. This single sample method is especially applicable to large scale investigations, acute and chronic, where evaluation of hepatic microsomal function is needed, but time and sampling facilities are limited.