Gonadal function in mink under artificial photoperiods.

Exposure of juvenile minks to artificial light conditions imitating early autumn (additional illumination from 20 June to 20 July followed by 8 h daylight from 21 July to 10 October--photoperiod I; 8 h daylight from 21 July to 10 October--photoperiod II) induced the prolonged modification of sex steroid concentrations. The increase in plasma testosterone concentrations during the prepubertal period was followed by a decrease before and during the mating period in experimental males. The concentration of oestradiol in plasma increased just before the start of the mating period (January and February) in females under photoperiod I, and these hormonal changes were accompanied by an increase in fertility. The conditions of photoperiod II (involving fewer hours of daylight) had less effect on the sex steroid concentration in the blood of females, and fertility was not changed. The response of gonads to hCG injection at the beginning of puberty (November) was higher in males exposed to photoperiod I conditions and in females exposed to both photoperiods than in control animals.