Genetic and physiological studies of ovulation rate, both natural and induced, in a random bred population of laboratory mice led to the following results.
(1) The heritability of natural ovulation rate in nulliparous females was 22±19%, estimated from the correlation between paternal half-sibs.
(2) Selection applied to natural ovulation in primiparous females led to a response in both directions, with a realized heritability of 31% (h2 within litters = 0·18 ± 0·013). The number of eggs shed by the High, Control and Low lines after 12 generations were respectively 21, 16 and 14.
(3) Selection applied to ovulation induced by 4 i.u. of PMS led also to a response in both directions, with a realized heritability of 22% (h2 within litters = 0·11 ± 0·038). The induced ovulation rates of the High, Control and Low lines after 12 generations were respectively 29, 19 and 14 eggs.
(4) PMS-equivalents were calculated from dose-response curves, and were found to be nearly the same in all lines except the line selected for high natural ovulation rate, which had a PMS-equivalent nearly double that of the Control. From this it was inferred that the increase of ovulation rate resulting from selection for high natural ovulation was due to an increased FSH activity, whereas the changes of ovulation rate resulting from selection for low natural ovulation and for both high and low induced ovulation were due to changes of ovarian sensitivity.
(5) Genetic correlations, estimated from correlated responses to selection, were: (i) between natural and induced ovulation in primiparous females, 0·33; (ii) between primiparous and nulliparous females in natural ovulation, 0·46; (iii) between primiparous and nulliparous females in induced ovulation, 0·82.
(6) The selection for induced ovulation produced changes in litter size following natural ovulation that were nearly equal to the changes in natural ovulation rate. The selection for natural ovulation, however, produced no clear changes of litter size.