The histidine requirement of growing kittens was determined from an experiment in which forty-eight kittens were randomly allocated to six amino acid-based diets supplying: 1.0, 1.5, 2.0, 2.5, 3.0 or 4.5 g histidine base/kg diet. 2. By 48 d it was obvious that 1.0 and 1.5 g histidine/kg diet were grossly inadequate so the kittens receiving these two diets were removed from the experiment. The other four groups of kittens continued to receive their diets for a total of 128 d. 3. Mean daily weight gain, nitrogen retention and food intake attained plateau values at 2.1 g histidine/kg diet. 4. Blood samples taken at 25 and 48 d after kittens were given the diets showed a significant effect of dietary histidine on haemoglobin (Hb) concentration. Hb and packed cell volume (PCV) attained asymptotic values at 3.0 g histidine/kg diet at 48 d. At 128 d, kittens consuming diets containing 2.0-4.5 g histidine/kg had similar Hb and PCV values. 5. Cataracts of both eyes were observed in two of nine female kittens which had received diets containing either 2.0 or 2.5 g histidine/kg. 6. A concentration of 3 g histidine/kg diet is recommended as a practical guide for feeding kittens. 7. There was a rectilinear relation (r2 0.99) between the logarithm of the histidine concentration of plasma and the concentration of histidine in the diet over the range 1.5-3.0 g histidine/kg diet.
