It has recently been shown that gossypol, a component of the cottonseed, has clear antifertility properties. The present study was planned to determine whether diets containing cottonseed in different proportions and in nontoxic amounts, could produce infertility in male rats. Diets were formulated with different concentrations of whole cottonseed flour and stock diet (Purina Chow). After 4 weeks of feeding these diets, the male rats were mated with virgin female rats in order to test male fertility rate. Thereafter, the male rats went into a recovery period during which they were fed stock diet for 6 weeks and were again mated after 3 and 6 weeks on this diet. The results show that: 1) infertility was produced by cottonseed flour diets even with a low concentration of gossypol (defatted cottonseed flour); 2) this effect was reversible at least after 6 weeks of gossypol-containing diets, even in the groups fed diets with a high gossypol concentration. The gossypol present in the diets decreases the number and motility of spermatozoa, even when present in concentrations below its toxic level, which may have an important deleterious effect on animal reproduction, since cottonseed is frequently used in the preparation of balanced diets mainly for animal consumption.