The effects of pinealectomy on the weight, the in vitro biosynthesis of steroids and the testosterone content of the duck testis (Anas platyrhinchos) were studied during an annual cycle. In control animals the testicular weight increased up to 12 times from the nonbreeding to the breeding season. Progesterone-4-14C was metabolized to 17α-hydroxypregn-4-en- 3,20-dione, androstenedione, testosterone, 5αpregnane- 3,20-dione, 5α-androstane-3,17-dione, 20α-hydroxypregn-4-en-3-one and 17α,2Oα-dihydroxypregn- 4-en-3-one. Pregnenolone-4-14C was converted to the same products plus progesterone, 3β, 17α-dihydroxypregn-5-en-20-one and dehydroepiandrosterone. All the enzymatic activities were localized in the microsomal fractions. The in vitro percent conversion of both substrates was higher during the non-breeding season, with the exception of the 20α-reduced metabolites. There was a close correlation between the evolution of the testicular weight and the testosterone content of the testis, which varied from 87 ± 7 ng (mean ± SE) in the testicular resting time to 170 ± 8 ng, during the breeding season. Pinealectomy at the beginning of spring produced an inhibition of the testicular weight, testosterone content and in vitro biosynthesis of steroids, except for the 5α-reductive pathway, which was stimulated. No differences were observed beyond the breeding season among pinealectomized, sham-operated and control ducks, either in the seasonal involution or in the new reactivation of the testicular function. (Endocrinology89: 1082, 1971)