1. In the seasons of 1957 and 1958–59 a total of 130 ewe lambs were used to investigate the effects on growth and sexual development of high and moderate planes of nutrition during the rearing period.2. The nutritional treatments imposed had significant effects on rate of growth; puberty occurred at an earlier age in high-plane than in moderateplane lambs. Also, in 1958–59, four moderateplane lambs failed to exhibit oestrus during their first winter of life.3. Puberty occurred at a significantly lower live weight in moderate-plane than in high-plane lambs.4. In lambs slaughtered after first oestrus, ovulation rate was not significantly affected by nutritional level, nor was the number of ovarian follicles greater than 2 mm. in diameter. However, high-plane lambs tended to have more follicles less than 2 mm. in diameter than moderate-plane lambs.5. In 1958–59 the ovarian response to a subcutaneous injection of 800 i.u. of p.m.s. was investigated in half the lambs from each nutritional treatment. Injections of p.m.s. caused a marked increase in ovulation rate over the uninjected control level but there was no difference in response between nutritional treatments. Pregnant mare's serum did not alter the size distribution of ovarian follicles.
Ovarian follicles during infancy in Romanov and Ile-de-France ewe lambs
