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The serum concentrations of testosterone and oestrogens were determined in stallions classified as geldings, normal (according to age) or infertile (azoospermic). There were significant differences in testosterone and oestrogen levels between the groups. Normal concentrations of testosterone and total oestrogens were attained after 16 months of age. Castrated and immature horses (< 15 months of age) had lower levels of testosterone and total oestrogens than did mature and infertile stallions. Azoospermic stallions had lower levels of testosterone and total oestrogens than did normal mature horses except in one case (B4) in which the level of total oestrogens was increased markedly. Critical ellipses for testosterone and total oestrogens were obtained for both groups (6-15-month-old colts and stallions aged > 4 years). The results suggest that serum levels of testosterone and total oestrogens may provide a sensitive index of the endocrine function of the testis in male horses.

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