A personal computer based program (DDAPD1) was developed for calculating or determining the apparent distribution function of a drug to a possibly heterogeneous biophase, the mean transit time of a drug at the biophase, the ratio of distribution clearance to biophase volume of distribution, the drug concentration-time function at the biophase (Cb(t)), and a function which reflects the relationship between the pharmacodynamic response and Cb(t) of the drug. This program uses an approach based on disposition decomposition analysis and the hysteresis minimization principle. The application of the program is demonstrated using published data for heptabarbital.