A new version of MULTI (ELS) written in BASIC was presented for population pharmacokinetics for microcomputers. The program is provided with four algorithms of extended nonlinear least squares (steepest descent method, quasi-Newton method with DFP formula, quasi-Newton method with BFGS formula and simplex method). Four transformations of parameters can be selected to impose constraints on the parameters (no constraint on a parameter. Pi = Qi2, Pi = B + (A--B) X sin2(Qi) and Pi = B + (A--B) X exp(Qi)/(1 + exp(Qi)), where Pi denotes a population mean parameter or variance of inter- and intra-individual variations, Qi is a intermediate parameter, A and B are lower and upper limits of Pi. The definition of a population model and the modifications for BASIC compiler in the new version of MULTI (ELS) are the same as in the old version.