The effects of the inclusion of raw and autoclaved whole faba beans (Vicia faba; RFB and AFB respectively) or faba bean fractions (cotyledons and hulls) in diets for growing broiler chickens (0–4 weeks of age) on performance, intestinal physiology and jejunal histological structure have been studied in three experiments. Significant decreases in body-weight as well as lower food consumption and higher food intake:weight gain ratio were observed in those animals fed on diets containing 250, 350 and 500 RFB'kg in the diet. Birds fed on AFB diets (500 g/kg) had significantly greater body-weights than chicks fed on RFB or raw faba bean cotyledons (RC). Significant increases in the relative lengths of duodenum, jejunum, ileum and caeca, pancreas relative weight, and intestinal transit time of birds fed on diets containing 250,350 and 500 g RFB/kg compared with control birds were observed. Including AFB (500 g/kg) in the diet significantly increased body-weight and significantly decreased pancreas weight compared with RFB (500 g/kg)-fed birds. The inclusion of RFB hulls had no effect on these variables. Dehulling or autoclaving of faba beans, or both, proved to have no significant effect on relative lengths of duodenum, jejunum, ileum and caeca, nor on caecal volatile fatty acid concentration in birds fed on 500 g faba beans/kg diet. Electron microscopy of the jejunal mucosa revealed discrete hyperplasia of polysomes and mitochondria1 swelling in those animals fed on AFB (500g/kg) or AC (4264g/kg). Pronounced strangulations were also observed along the microvilli, whose length was similar to that of control birds. The inclusion of RFB hulls, either autoclaved or raw, led to no ultrastructural changes in the enterocytes, as detected by electron microscopy. Birds fed on diets containing the cotyledons of RFB (RC, 426 4 g/kg) rather than whole RFB showed the same ultrastructural disorders as RFB (500 g'kg)- fed birds. The present study shows that factors other than those usually claimed, i.e. protease inhibitors, phytates, tannins and lectins, may be contributing to the low nutritional value of V. faba seeds for growing chickens.