STResS (Simulated Toxicant-Related Stress) is a program written in DEC FORTRAN v. 6.2. This program can be run either interactively or batch mode. This program is designed to model the effects of toxicant exposure on a simulated population of a specific species, as well as the effects of the toxicant on the demographic and genetic characteristics. The toxic effect on the time-to-death is based on an accelerated failure time model in which the time-to-death depends on size, sex and genotype, toxicant concentration, and frequency and duration of exposure. Sexual, fecundity, and meiotic drive/gametic selection can also be included. Multiple simulations can be run for a user-specified number of gestation periods of user-specified length. The effect of winter can be included, and the exposure duration can be changed once during each simulation, if desired.