A new software package, NONLIN84, has been developed for the analysis of general nonlinear models including pharmacokinetic models. NONLIN84 is easier to use than the older NONLIN77, and can handle a wider class of estimation problems, such as maximum likelihood estimation involving iterative reweighting. Two large libraries of pharmacokinetic models are distributed with NONLIN84 and can be accessed by simply specifying a model number. A companion program, PCNONLIN, runs on DOS based microcomputers and retains most of the features of NONLIN84.