The carcinogenicity of maleic hydrazide is discussed by several national and international organizations because of contradictory results of a number of carcinogenicity studies carried out in the past. Because maleic hydrazide is used in agriculture on edible crops, an oral carcinogenicity study with rats was carried out for 28 months at dietary levels of 0, 1.0 and 2.0% maleic hydrazide which contained less than 1.5 mg hydrazine/kg product as impurity. In this study as well as in an experiment with mice carried out with the same batch of maleic hydrazide at the International Agency for Research on Cancer (IARC) in Lyon, France, treatment did not affect tumor incidence and it was concluded that maleic hydrazide itself is not a carcinogen. Most likely the presence of relatively high levels of hydrazine as an impurity was responsible for the contradictory results in studies as reported previously. Furthermore the results of this study showed that 1.0 and 2.0% maleic hydrazide in the diet caused proteinuria and increased protein/creatinine ratio's in the urine in both sexes without detectable histopathological changes in kidney or urinary tract. From this study, based on the effects of kidney function the "no-toxic" effect level is considered to be lower than 1.0% maleic hydrazide in the diet of rats.