Guinea-pigs are usually mated in permanent polygamous groups, one or more males with a large number of females, the young being harvested when ready to wean. This system is economical of labour, and is efficient in allowing of mating at post-partum oestrus and therefore of maximum reproductivity. There are, however, serious disadvantages; it is difficult to keep track of individual performance and difficult therefore to eliminate unproductive sows, the ancestry and exact age of the young are often unknown, and the conditions of rearing young among a crowd of adults are probably not ideal. These considerations suggest that, given high-grade breeding stock, the polygamous system is adequate, and probably inevitable, for the mass production of young for experimental purposes, but that it leaves much to be desired for the production of breeding stock. The obvious solution of this difficulty is to maintain a large polygamous and anonymous colony for mass breeding, and, quite separately, a small closely recorded colony to supply the breeding animals. A goodsow should produce at least 10 young per year in a life of 2 years. A demand of 10,000 animals per year, for experimental work, therefore requires a mass-breeding stock of 1000 sows, an annual replacement of 500 sows and a stock-breeding stock of 100 sows. According to our experience, about 2 hr. daily on the part of a trained technician would be required to maintain close recording of such a pilot stock and of its progeny to puberty (Bruce & Parkes, 1947b).