The estuarine myside, Mysidopsis bahia, was reared through an entire life-cycle in a range of endrin concentrations. Physiological rates of ammonia excretion and oxygen consumption were measured for various life stages. These individual functions and those of growth and reproduction were combined into bioenergetic and physiological indices to help describe the chronic effects of endrin on mysids. Concentrations of endrin that were chronically lethal stimulated the respiration rates of newly released juveniles after only one day exposure. Sublethal exposure to endrin reduced growth rates of rapidly growing juveniles.