Information relating to post-partum mating has been described in guinea-pigs kept in six small colonies containing variable numbers of the sexes running together continuously in floor pens of standard size. Litters were born on the floor and rearing was communal (Method 1). Three other like colonies were similarly maintained, except that the females in late pregnancy were separated from the male to avoid post-partum mating (Method 2). The conditions of farrowing and rearing were similar to those of Method 1. The data given by thirty-four sows of a closely recorded (stock) colony bred in units of six females and one boar, but in which every litter was born and reared under ‘cage’ conditions (Method 3), were used for comparative purposes only.

Post-partum mating was considered to have occurred only when the interval between the births of any two consecutive litters of the same sow did not exceed 70 days. Of litters born under Method 1, 80% were conceived at post-partum oestrus; the mean gestation period was 68 days. The average inter-parturition interval of all sows breeding under Methods 1 and 2 was 74 and 118 days respectively. The optimal ratio of the sexes in a colony breeding by both methods was judged to be twelve females to one male.

Litter size varied between 3·98 (Method 1) and 3·69 (Method 3) and by all three breeding methods, the proportion of young already dead or that died at birth was about 5%. Deaths were more common in summer months when litter size was maximal.

The post-natal death-rate during the first 28 days of life among young reared communally (Methods 1 and 2) was 10% and was fivefold that observed in litters reared separately by their mothers (Method 3). This death-rate was highest during the winter months due to the difficulty of providing adequate amounts of fresh green vegetables.

The annual output of young guinea-pigs from colonies bred under Method 1 was 62% greater than that from other colonies bred by Method 2. The more rapid sequence of littering did not affect the quality (body weight) of the young at birth or at weaning.

Attention has been drawn to a possible deleterious effect of intensive breeding to the health of the females and their lessened resistance to infection. It has been suggested that, for this reason, the breeding life of the sows should be limited to 1 year.