For high-throughput screening in drug development, methods that can reduce analytical work are desirable. Pooling of plasma samples from an individual subject in the time domain to yield a single sample for analysis has been used to estimate the area under the concentration-time curve (AUC). We describe a pooling procedure for the estimation of the area under the first moment curve (AUMC). The mean residence time (MRT), and where intravenous dosing has been used, the steady-state volume of distribution can then be determined. Plasma samples from pharmacokinetic studies in dogs and humans analyzed in our laboratory were used to validate the pooling approach. Each plasma sample containing a prokinetic macrolide and three of its metabolites was first analyzed separately, and AUCs and AUMCs were calculated using the linear trapezoidal rule. The procedures for the estimation of AUC by sample pooling have been reported by Riad et al. [Pharm. Res. (1991) vol. 8, pp. 541-543]. For the estimation of AUMC, the volume taken from each of n samples to form a pooled sample is proportional to t(n)(t(n+1) - t(n-1)), except at t0 where the aliquot volume is 0 and at t(last) where the aliquot volume is proportional to t(last)(t(last) - t((last)-1)). AUMC to t(last) is equal to C(pooled) x T2/2, where T is the overall experimental time (t(last) - t0). The ratio between AUMC and AUC yields the mean residence time (MRT). Bivariate (orthogonal) regression analysis was used to assess agreement between the pooling method and the linear trapezoidal rule. Bias and root mean square error were used to validate the pooling method. Orthogonal regression analysis of the AUMC values determined by pooling (y-axis) and those estimated by the linear trapezoidal rule (x-axis) yielded a slope of 1.08 and r2 of 0.994 for the dog samples; slope values ranged from 0.862 to 0.928 and r2 values from 0.838 to 0.988 for the human samples. Bias, expressed as percentage, ranged from -25.1% to 14.8% with an overall average of 1.40%. The results support the use of a pooled-sample technique in quantitating the average plasma concentration to estimate areas under the curve and areas under the first moment curve over the sampling time period. Mean residence times can then be calculated.