To estimate the tryptophan requirement of the kitten, six male and six female kittens were presented diets containing 0.7, 0.9, 1.1, 1.3, 1.5 and 3.0 g tryptophan/kg diet for six experimental periods lasting 10 d in accordance with a 6 X 6 balanced Latin-square design. Mean daily weight gain and nitrogen retention (N intake minus urinary and faecal N) plateaued at 1.1 g tryptophan/kg diet indicating that the minimal tryptophan requirement of the kitten was 1.1 g/kg diet. Plasma tryptophan concentration exhibited a marked increase at dietary tryptophan concentrations above 1.3 g/kg diet.