Groups of 90 (control) and 54 (treated) rats of each sex were given amaranth in their diet to provide daily intakes of 0 (control), 50, 250 or 1250 mg/kg for 111 wk (male) and 112 wk (female) after weaning. The rats had also been exposed to the same dose levels in utero, and their parents were exposed for 60 days before mating. The colouring had no adverse effects on fertility, haematological parameters, serum chemistry or incidence of tumours. All treated animals showed contamination of the fur and red colouring of the faeces and at the high dose only the faecal pellets were poorly formed. Rats in the high-dose group produced more pups, and the average pup weight was lower than that of the controls. Rats of the F1 generation given the highest dose level were slightly lighter than the controls despite a small increase in food and water intake. Both sexes given the highest dose level and males given 250 mg/kg/day had increased caecal weight. High-dose females excreted more protein in the urine after 18 months and on histopathological examination females in all treated groups showed an increased incidence of renal calcification and pelvic epithelial hyperplasia with degenerative changes. It is concluded that amaranth fed to rats at dose levels of up to 1250 mg/kg/day in the diet did not have any carcinogenic effect. However, because of the effect on the kidneys of the females it was not possible to establish a no-untoward-effect level in this study.