Concentrations of D-Asp, NO levels and titers of testosterone in the testes of A. platyrhynchos during the reproductive period (RP) and non reproductive period (NRP). D-Asp concentration was higher in RP, corresponding to maximum testosterone titre and to minimum NO level. Each point represents the mean value ± S.D. from six individual ducks. **, p < 0.01, Reproductive versus Non Reproductive period.