Experimental lesions in the ventro-medial hypothalamus of the adult rat produce a type of obesity associated with considerable gains in body weight (Hetherington & Ranson, 1940) and body fat (Montemurro & Stevenson, 1957). However, the destruction of the ventro-medial hypothalamic area in weanling rats rarely results in increased body weight (Bernardis, Box & Stevenson, 1963; Han, Lin, Chu, Mu & Liu, 1965; Bernardis & Skelton, 1966). Yet body composition studies by Han et al. (1965) have shown that weanling rats can become obese. A rapid means of determining obesity was described by Hetherington (1943) and has been recently revived by Szentagothai, Flerko, Mess & Halasz (1962) and Bernardis & Skelton (1966, 1967). It consists of dividing the cube root of the body weight by the naso-anal length. The resulting figure, referred to as 'Nutritive Ratio' (Lee, 1928) or 'Lee Index' is a measure of obesity. A figure of about 0·300