A diet which contained less than 1 microgram Zn per g was eaten readily by guinea pigs. No signs of zinc deficiency were produced unless the animals were deprived of access to zinc-supplemented solid food from birth and were maintained on grid floors to prevent coprophagy or ingestion of sawdust bedding material. At an early stage of deficiency, plasma zinc concentration, food intake and growth were reduced and large regular variations of food intake (food cycling) were observed. More severe deficiency produced decreases in the zinc content of other tissues and characteristic epidermal lesions. The requirement of guinea pigs for zinc for growth is probably about 2.5 micrograms per g feed which is much lower than for calves, rats and pigs.