This paper presents a computer program, MODFIT, written in FORTRAN, primarily for use on the Digital Equipment Company VAX series computers for the mathematical analysis of concentration-time data. Drug data generated from biological fluids and tissues may be fitted by a variety of different models. For many models, parameter starting estimates are program generated prior to automatic nonlinear regression analysis using a modified Davidon-Fletcher-Powell algorithm. The output of results is extensive and plotting facilities are available. Explicit and differential equation models may be fitted to single dose data and simulations using all models (single or repeat dose) may be employed to generate drug concentration-time data with plotting output. The package has been tested on numerous data with no problems regarding local function minima. Some comparisons with existing programs have been made and MODFIT compares well with respect to robustness, efficiency, and ease of use.