Plasma and pituitary levels of LH in field voles, Microtus arvalis, reared under two different photoperiods.

10 Plasma LH levels were measured in voles reared under long (15 L: 9 D) or short (10 L: 14 D) light photoperiods by radioimmunoassay from a Rat LH-Anti Ovine LH system. 20 The number of animals exhibiting a detectable LH level (i.e. greater than 1 ng per ml) and the mean plasma LH values were higher for long days than for short days in spite of a very large variability between animals. 30 The sensitivity of the hypophysis towards LH. RH increased after gonadectomy, but is not controlled by the daily length of the light photoperiod even in the castrated animals.