Lactation lasts for about 14 days in the field vole. Early in lactation the corpora lutea were large although some signs of degeneration were present; by Day 13 none could be recognized. Graafian follicles were present throughout the period but were slightly reduced in size during mid-lactation.
Vaginal smear patterns varied considerably between individuals although nucleated epithelial cells usually predominated.
Mating tests were carried out between Days 2 and 13 of lactation. 81% proved successful; coitus occurred on all days. 59% of the matings resulted in pregnancies that were carried to term. No reduction of litter size was apparent as a result of these matings. Gestation length was slightly prolonged when more young were being suckled. Most matings took place when there was a predominantly nucleated epithelial smear.
It is concluded that, in this stock, the only corpora lutea present during lactation are those of the previous pregnancy. The presence of Graafian follicles suggests that suckling does not entirely inhibit fsh secretion although the decrease in follicular size in mid-lactation may indicate slight fsh reduction. There were no 3- to 4-day oestrous cycles. The occurrence of mating and the predominant type of vaginal smear suggests continuous secretion of oestrogen. This may partly explain the lack of a marked implantation delay during lactation. There appears to be no inhibition of release of ovulatory-inducing hormone due to suckling nor of ovulation and resulting gestation even though the Graafian follicles are slightly reduced.