Plasma immunoreactive inhibin (i-inhibin) was measured in Assaf and Booroola-Assaf (1/2 Booroola-1/2 Assaf) ewe lambs between the ages of 2 and 15 weeks and correlated with plasma FSH. I-inhibin concentrations were similar in both genotypes and was maximal at 2 weeks of life and decreased thereafter. These levels (mean +/- s.e.) were 1.28 +/- 0.12 and 1.14 +/- 0.10 ng/ml at 2 weeks and 0.55 +/- 0.06 and 0.45 +/- 0.05 ng/ml at 15 weeks of age in Assaf (n = 13) and Booroola-Assaf (n = 13) respectively. In Assaf ewes an inverse correlation between plasma inhibin and FSH was observed at 4 and 15 weeks but not at 2 weeks of age. In Booroola-Assaf ewes, no significant correlation was found at any age studied. At 4 weeks of age, Assaf and Booroola-Assaf ewe lambs had very different ovarian morphology. Assaf lambs had large ovaries with a mean diameter of 1.00 +/- 0.09 cm (mean +/- s.e.) and many preantral and antral follicles present, while Booroola-Assaf had much smaller ovaries (0.66 +/- 0.03 cm, p < 0.05) with only a few antral follicles. It seems likely that the FecB allele from the Boorola Merino influenced the structure of the ovary although the contribution of other Booroola genes cannot be ruled out. In summary, the plasma i-inhibin concentrations decreased between 2 and 15 weeks of age irrespective of genotype. It is speculated that in prepubertal ewe lambs the ratio of bioactive to immunoreactive inhibin varies with age and between the genotypes with different ovarian development.