Male rats of proven fertility were fed the following diets for 28 d either with or without 0.075% 5-thioglucose (5-THG): AIN-76 diet (A76): a diet with 13% casein, 2% glucose and the balance of the calories as free corn oil fatty acids from corn oil (2G); and a similar diet, isocaloric with 2G, with the glucose level increased to 20% (20G). The diets alone without 5-THG had no influence on any of the parameters measured. Body weight gain was lower in rats fed diets containing 5-THG than in those fed diets without 5-THG. In rats fed A76, the only 5-THG effects on male reproductive tract (MRT) tissues was the appearance of testicular multinucleate giant cells (MGC). In rats fed either 2G or 20G, the MRT effects of 5-THG included the appearance of MGC, a lower number of germ cells at most stages of maturation, lower sperm counts and biochemical changes in testis slices and in germ cell preparations compared to rats not fed 5-THG. There were fewer Step 7 spermatids in rats fed 5-THG in 2 G than in those fed 5-THG in 20G. It is concluded that the MRT toxicity of 5-THG is influenced by diet, being potentiated by the low protein diet high in free fatty acids and, to a lesser extent, by low glucose levels within these diets.