The plasma clearance of [ 131 I]o-iodohippurate, which is an estimate of effective renal plasma flow, can be measured from one or two blood samples drawn after an i.v. bolus of tracer. The most widely used method is based on a single sample. Here, it is shown that the error (relative to a complete curve of six or more blood samples) can be halved by using two samples rather than one. Since the one-sample method is satisfactory for most clinical purposes, the two-sample method is recommended only when special accuracy is needed.