The algorithm of this program for estimating rate constants of drug absorption is mainly based on the Loo-Riegelman method. In order to improve the flexibility of the program, several options for the users were included: one-, two- as well as three-compartment open models with first-order elimination; two optional methods to calculate the area under the curve, i.e. the simple trapezoidal method and the Lagrange method combined with log-trapezoidal approximation and others. In addition to the estimation of the rate constant of drug absorption, this program can be applied for the design of optimal sampling schemes. For this purpose an extra subroutine for simulating plasma drug concentration-time courses is also included.