Xueying Yu, Yujiang Hao, Brian CW Kot, and Ding Wang (2016) Photoperiod plays an important role in the seasonal regulation of reproduction in mammals. In the present study, we examined the effect of an extended photoperiod (light/dark: 14 h/10 h) during the usual sexually inactive phase (Jan-Mar) on testicular sonographic appearance and sexual behaviors of two captive Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis, YFP). During the extended photoperiod treatment, the parenchyma pixel intensity, cross- sectional area of testicular sonographic images, as well as the frequency of sexual behaviors of the two male porpoises increased significantly compared to the control treatment. Parenchyma pixel intensity, the cross- sectional area of testicular sonographic images and the frequency of sexual behaviors of the two captive males all increased in response to extended photoperiod. These results may suggest that photoperiod is a potent factor regulating the reproductive seasonality in YFPs.