Genetic differences in fecundity have arisen as a result of selection for litter size in Merino sheep since 1954. The selection experiment involves three flocks designated 'O', 'T' and 'B' and their genetic history has been described by Turner (1969) . In the 'O' flock, in which selection has been against multiple births, the ewes regularly have a low (0 to 10%) incidence of twin births. The 'T' and 'B' flocks have a medium (50 to 60%) and high (70 to 80%) incidence of multiple births. In the

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... ' flock, about 30% of ewes have triplets and 10% have quadruplets or quintuplets each year. Current research on these groups is designed to examine the endocrine changes that have accompanied the genetic divergence in fecundity. It is known that ewes from the flocks of high fecundity have a higher ovulation rate and shed more eggs in response to exogenous gonadotrophin (Bindon, Ch'ang & Turner, 1971). The present report deals with the plasma LH levels of young lambs from the three groups. Peripheral plasma samples were collected between 09.00 and 10.00 hours on about the 30th day of life (range 27 to 33 days) and LH was measured by radioimmunoassay (Goding & co-authors, 1969). The results were expressed in terms of ng NIH-LH-S16 per ml plasma. In this report, data from male and female lambs have been pooled, although there has been a slight but consistent difference in LH levels in favour of ewe lambs. In 1970, only female lambs were examined and a sample only of the three populations included. In 1971 and 1972, all lambs of both sexes were studied.