The amino acid composition of the diet ingested by reference and cafeteria diet-fed rats has been analyzed in Wistar rats from day 30 to 60 after birth. Body protein amino acid composition and the urinary and faecal losses were also measured. Cafeteria diet resulted in a higher proportion of amino acids extracted from the diet, although this diet had a very similar amino acid composition to that of the standard reference diet. The net rates of amino acid accretion into body protein were similar for cafeteria and reference diet-fed rats, resulting in a comparable net overall accumulation of protein. Urinary losses of amino acids were small, but higher for reference diet-fed rats. Cafeteria feeding leads to an essentially equal amino acid intake pattern to that resulting from the reference diet. In addition, cafeteria-feeding resulted in a similar amino acid nitrogen intake and practically equal amino acid availability, which is translated into higher net protein accrual and lower nitrogen losses in cafeteria-fed rats. It is postulated that the lower protein-energy proportion of the cafeteria diet--and not its amount in absolute terms--could trigger a series of amino acid-sparing mechanisms that eventually result in even higher amino acid availability, which leads to increased net protein deposition and a wider nitrogen gap.