Velocity of clearance of indocyanine green (ICG) is used for the measurement of liver function. Thirty-nine curves of ICG concentration vs. time, from 24 rats, were evaluated for the best method for the determination of the initial velocity of clearance of ICG. A bolus dose of ICG (0.66 ± 0.10 mg/kg) was given, and [ICG] was recorded once per second for the 270 seconds from 30 seconds to 5 minutes after ICG administration with the aid of computer-assisted data acquisition. The 39 observed curves were then fit to six equations: a second- and third-order polynomial, the mathematical constant e raised to a first- (first-order kinetics), second-, and third-order polynomial, and an integrated form of the Michaelis-Menten equation. The residual errors of the fitted equation to the observed data were evaluated. First-order kinetics and an integrated form of the Michaelis-Menten equation were the least accurate methods of describing the [ICG]-time curve. The curve was better described by three equations: a third-order polynomial, e raised to a second-order polynomial, and e raised to a third-order polynomial. We recommend that one of the latter three equations be used in the determination of the initial velocity of clearance of ICG.