An in vitro method of evaluating the environmental stability of a drug under varying physical parameters was developed. The stability of sulfadimethoxine and ormetoprim in a 5:1 ratio in an aquatic environment under varying pH's, temperatures, and salinities was determined. The effects of quartz sand, bentonite clay, high density poly-ethylene, and poly-vinyl-chloride on the disposition of these drugs in the aquatic environment were also evaluated.

Sulfadimethoxine and Ormetoprim concentrations were shown to be stable for a period of one year at the salinities and at pH's tested. Ormetoprim concentrations remained stable at the temperatures tested. Sulfadimethoxine concentrations were unaffected by warm temperatures but were decreased by low temperatures. The sulfadimethoxine concentration did return to it's original level upon warming the solution. The concentrations of the two drugs were unaltered by the presence of high density poly­ethylene, poly-vinyl-chloride, and quartz sand. The addition of bentonite clay resulted in a decrease in the ormetoprim concentration while the sulfadimethoxine concentrations were unchanged. The ormetoprim levels were returned to their original levels by acidification of the solution. The highly stable nature of these two drugs precluded the determination of an environmental half-life for either drug. The data acquired indicates the environmental half-lives of these drugs must exceed one year and are very likely to be several years in duration.