METHODS Animals The rats were a hooded strain which originally came from the Lister Institute. They had been kept as a closed colony for some years. Initially animals for parabiosis were more closely inbred by mating litter-mates for three generations before using the offspring; later pairs were made from ordinary stock litters. In order to standardize the size of the animals as far as possible they were bred from females weighing between 200 and 250 g, and only second and third litters which con-tained 8-12 young were used. The young were separated from the mothers on the 21st day after birth. The diet was Mill Hill Diet 41 (Bruce & Parkes, 1949). Food and water were provided ad libitum. When rats had been subjected to hypothalamic lesions, food was scattered around the cage as well as supplied in the usual hoppers.

Parabiosis The pairs of rats united in parabiosis were litter-mates of the same sex whose weights did not differ by more than 3% at the time of joining. Most pairs were united when 4 weeks old, but the most recent ones have been united when a week or two older. As many pairs as possible were made from each litter, and any compargle animals left over were kept as single controls. Some of these single animals were subjected to sham operations, in which equally extensive surgery was carried out without uniting animals. Both sexes were used. The anaesthetic was pentobarbi-tone sodium, given intraperitoneally in a dose of approximately 6 mg/100 g body weight. The surgical technique used for the earlier pairs was that of Bunster &Meyer (1933), in which the peritoneal cavities are opened and the four cut edges ofmuscle and peritoneum united in one suture. In later pairs the upper and lower cut edges were sutured separately, making a'coelio-anastomosis'(Sauerbruch & Heyde, 1908). This avoids the risk of small openings forming between the peritoneal cavities and occasionally causing death through strangulation of loops of intestine. In the most recent pairs the abdominal cavities have not been opened and firm union of the posterior parts of the bodies has been secured by tying the femora together (JM Ledingham, personal communication). Bunster & Meyer's technique of uniting the scapulae was followed, except that the scapulae were scraped to expose raw bonebefore suturing. Aseptic technique was used as far as practicable.