Repeated sampling of a drug solution that is recirculated through a perfused body increases the rate of drug disappearance from the perfusate. When the volume of the drug solution (VT) is maintained constant by addition of drug-free perfusate after sampling, the measured drug concentration (Ci) can be corrected for drug removed in previous samples by using the equation C'i = CiVTC'i-1/(VT - VS)Ci-1, where C'i is the corrected drug concentration in the ith sample, VS is the volume of the sample, and C'1 = C1. An error in an particular Ci is not transmitted to a subsequent C'i value. The method can be used when the time interval between samples and when VS vary from sample to sample, but return of the drug from the perfused body to the perfusate after sampling may cause C'i to be overestimated.