Abstract
Day length or photoperiod is an important determinant of seasonal breeding or sexual maturation in the vole (Microtus agrestis). Exposure of voles to short (6 h) photoperiods slows gonadal development or causes active gonads to regress. Abolishing pineal activity either by chemical sympathectomy, using 6-hydroxydopamine (6-OHDA), or by surgical pinealectomy, prevents this inhibiting or regressing effect of short photoperiod upon gonads. Thus, the pineal gland appears to play a part in the measurement of day length by voles and in the physiological causation of their seasonal breeding.