1. Twenty-eight high grade Sussex X Afrikander heifers, maintained in an artificial environment, enforcing dry rations and restricted exercise, has been observed regarding (1) the length of the period between birth and first oestrus, and (2) the length of the dioestrous cycle.
2. The average age at sexual maturity, or the appearance of the first oestrus, was 628·5 days. Sixteen heifers born during the warm season showed oestrus for the first time at an average age of 553 days. Twelve heifers born during the cold season showed the first oestrus at an average age of 496 days.
3. There was a striking difference in the age of sexual maturity of the heifers born in different years. This difference cannot be explained as all the heifers were kept under similar conditions.
The influence of rainfall on sexual maturity was not significant, neither could hereditary influence be proved.
4. The heifers born from cows having access to unlimited sunlight and exercise showed the first oestrus on an average of 133 days earlier than heifers whose mothers were kept with a minimum of sunlight and exercise. However, in view of the fact that there was a significant difference in the age of sexual maturity of the heifers born from two groups of cows with unlimited sunlight and exercise, one group of which was mated six months later than the other, the difference of 133 days must be taken with reserve.
5. The average age of sexual maturity was, as a rule, much greater than that recorded in Europe than America.
6. The total number of dioestrous cycles recorded was 383. The average length of the dioestrous cycle was 20·3 days, with a range of 18 to 23 days for 349 cycles, or 91·12 per cent of the total.
There were 34 dioestrous cycles outside this range, or 8·8 percent. These were considered abnormal cycles.