Standard dataGrowth. Live-weight gains of kittens were first measured by Abderhalden (1898) who found that litters doubled their birth weight in 9 days, and could equal their mother in weight by the end of the lactation period. Abderhalden correlated the rapid growth of kittens with the high solid content of cat's niilk; according to Abderhalden (1898) and to Davies (1939) cat's milk contains: protein 7'0-7'3y0, fat 44-475%) lactose 4.8% and ash 0.58-1.02% by weight. Young kittens are continuous feeders, gaining steadily from birth, a fact observed by Latimer & Ibsen (1932) who followed the weight gains of twelve kittens for 13 weeks, and Hall & Pierce (1934) who studied the growth of seventy healthy kittens, born in the laboratory and reared to 7 weeks (the lactation period) under standard conditions. da Silva (195ob) gave figures for the weight gained by small numbers of kittens from 2 to 1 5 months of age collected from domestic homes and reared in the laboratory on a standardized ration. We measured the weight gains of healthy kittens born in our colony and reared on the stock diet described below (Dickinson & Scott, 1956~).Weighings to the nearest 2 g were carried out three times weekly, between meals, of which one was offered at I I a.m., the other at 4 p.m. ; unfed kittens were found to weigh about 5% less than those that had eaten recently. Growth curves (Fig.