Effects of age and naloxone on testicular function were studied in golden hamsters exposed to short photoperiods. Subjection of hamsters to short photoperiods of 6 h light: 18 h darkness for 6 weeks led to testicular regression in young adult (5-6 months) or middle-aged (11-12 months) golden hamsters but not in prepubertal hamsters of 1-2 months of age. The middle-aged hamsters had decreased testis width by week 4 of treatment and the young hamsters by week 5. Daily injection of naloxone at the time of 'lights on' partially prevented testicular regression in young and middle-aged hamsters but the extent of regression was significantly greater in the middle-aged animals. Plasma LH and FSH concentrations were significantly reduced in hamsters placed in short photoperiods regardless of age or testicular weight, while naloxone treatment significantly increased the LH concentrations in all age groups. Plasma beta-endorphin-like immunoreactivity was significantly increased by short photoperiod or older age. These results indicated that (a) the sensitivity of the testicular suppression to short photoperiod increases as a function of age, (b) naloxone, a specific opiate receptor blocker, can partially prevent short photoperiod-induced testicular regression and (c) ageing and short photoperiods increase beta-endorphin-like immunoreactivity. It is concluded that the opiate system may be involved in ageing and photoperiod regulation of reproductive function.