1. Diets containing a freeze-dried tannin extract from faba beans (Vicia faba L.) at concentrations of 0, 8 and 16 g/kg were fed to growing chicks and rats. 2. Chicks fed on the tannin-containing diets exhibited a high mortality and significant decreases in body-weight as well as lower food intakes and higher food intake:weight gain ratios (P < 0.01). In rats there was no mortality but the growth and food intake were adversely affected (P < 0.05). 3. Histological changes occurred in the ileal mucosa of chicks. Atrophy and shortening of villi with distortion of their architecture were observed. Similar histological disorders but less pronounced were also found in rats. 4. Histopathology of the liver of both chicks and rats showed an hydropic degeneration of hepatocytes, clearly more severe in the former than in the latter.