Representative immunohistochemical distribution and localization of NO synthase in the testes of duck A. platyrhynchos during the reproductive period (RP) ( A ) and the non reproductive period (NRP) ( B and C ). NOS positive cytotypes were found elsewhere in seminiferous tubules, but they were very rare in the interstitial tissue of RP (A). Immunopositivity was mainly localized in Leydig cells of the NRP (B and C). Histograms represent quantification of immunopositive Leydig cells/mm2 (D). **, p < 0.01 NRP versus RP. L, lumen. Scale bars: a, 100 μm; b, 50 μm; c, 20 μm.