Fig. 2. The effects of a daily pulse of monochromatic ultraviolet radiation at 320nm (Short + 320nm) on the reproductive and immune systems of developing juvenile male hamsters. (A) Mean testes mass (%) of animals exposed to a long photoperiod (16h:8h L:D), a short photoperiod (10h:14h L:D), or a short photoperiod with a daily 15min pulse of monochromatic UV-A light at 320nm during the middle of the night. (B) Mean thymus mass (%) of the same animals. Values are means ± S.E.M. (N=29). An asterisk indicates a significant difference (P<0.001) from the long-photoperiod value.