Twenty one pairs of twins, all progeny of one sire, of which one was a male and the other a female, were weighed monthly from 3 to 14 months of age. Scrotal circumference (SC) and testes v61ume (TV) in rams were measured simultaneously. A vasectomized ram was used to detect oestrous in ewes at 18 months of age. Laparotomy was used to determine the ovulation rate. Statistically significant (P<0,05) phenotypic correlations are only indicated between the ages of 5 and 6 months. Correlations between these ages varied from 0,371 to 0,481. At 5 months of age all these coefficients are statistically significant (r = 0,428 to 0,466; P<0,05), which coincided with the attainment of puberty. When testis size measurements were adjusted for differences in body mass, all correlation coefficients became statistically nonsignificant (r = 0,133 to 0,348; P>0,05). Partial correlation coefficients between testis size and ovulation rate when body mass in both rams and ewes were fixed, varied from 4 to 7 months of age in the ram from 0,203 to 0,616. These correlations were statistically significant for TV at 6 months (P<0,05) and for SC at 7 months (P<0,01). Although prepubertal testis size would be of more practical value in selection for ovulation rate, testis size at puberty may be more valuable as an indication of ovulation rate in female relatives