Adult male Syrian (golden) hamsters, maintained under either 22 ± 2 or 32 ± 2 °C, were treated with 8 or 11 weeks of exposure to either long photoperiod (14:10), short photoperiod (8:16), or to long photoperiod with a daily afternoon melatonin injection. By 8 weeks, the animals kept at 22 °C and treated with daily afternoon melatonin injection exhibited a dramatic reduction in testicular and accessory sex organ weight, but the animals kept at 32 °C and treated in the same way exhibited only slight decreases in testicular and accessory organ weights. Short photoperiod caused a slight decrease in testicular and accessory organ weights of hamster kept at 22 °C, while it had no significant effects on reproductive organ weights of the animals maintained under 32 °C. By 11 weeks, the daily afternoon melatonin injection elicited further reduction in testicular and accessory organ weights of the animals maintained under both 22 and 32 °C. However, the reduction in animals kept at 32 °C was not as great as that in animals kept at 22 °C. Although short photoperiod caused an obvious decline in reproductive organ weights of the animals at 22 ¤C, only a slight decrease was seen in hamsters at 32 °C. As with reproductive organ weights, testosterone levels were depressed more rapidly and completely in animals maintained at 22 °C. These results indicate that elevated ambient temperature changes the rate at which the gonads of hamsters regress in response to daily afternoon melatonin injections or short photoperiod. The daily afternoon melatonin injections and short photoperiod caused pituitary regression in animals placed in both temperatures after 8 and 11 weeks of treatment. Temperature had no influence on the effects of short photoperiod and melatonin injection on pituitary weight. Finally, plasma thyroxine (T4) levels and the free T4 index (FT4I) exhibited a reduction to melatonin injection under both the lower and higher temperatures. Short photoperiod caused a decrease of plasma T4 and FT4I in animals under 22 °C, while it only led to a lesser reduction of T4 and FT4I in animals under 32 °C. Therefore, the warmer temperature appeared to have slight influence on the response of plasma T4 and the FT4I to short photoperiod. Temperature showed a strong influence on the plasma triiodothyronine (T3) and the free T3 index (FT3I). A dramatic decrease in plasma T3 levels and FT3I was observed in animals maintained at elevated ambient tempe