102 John W. Everett 



workers had demonstrated that pentobarbital sedation of the cycling rat 

 during a certain "critical period" on the day of proestrus will predictably 

 block the expected ovulation for a 24-hr period. In rats that have been sub- 

 jected to controlled illumination of 14 hr daily the critical period extends 

 from 2.00 p.m. until after 4.00 p.m. It had first been defined by the use of 

 atropine, a potent and predictable blocking agent for ovulation in both the 

 rabbit and the rat (15,32,33). 



In the summer of 1958 R. L. Riley and J. W. Everett set out to repeat 

 Critchlow's experiment in a small series of rats, in preparation for attempts 

 to induce ovulation during pseudopregnancy. We were gradually led into 

 exploration of more and more rostral regions, eventually finding that the 

 preoptic area could give more predictable results than had been possible 

 in the tuberal region. We were briefly joined by Dr. C. D. Christian. During 

 the summer of 1959 J. R. Harp joined us and undertook a study of thresholds 

 in the preoptic region. 



METHODS 



The methods employed can best be described by giving an account of the 

 standardized procedures that were used in the late phases. Departures from 

 these standards will be noted whenever they are of significance. 



As in previous investigations in this laboratory we employed rats of an 

 inbred strain derived from Osborne-Mendel stock. The animals were 

 regularly cycling adult females characteristically in the age range of 5 to 10 

 months and weighing 200 to 250 g. It is our practice to maintain at all 

 times a group of 50 to 70 potential experimental subjects, from which vaginal 

 smears are prepared daily 6 days a week. As animals are removed for 

 experiments, others are added from time to time. Thus we have at all times 

 a large control group. In the selection of an experimental subject it is 

 demanded that she have a vaginal smear history of at least two 4-day cycles 

 or two 5-day cycles in sequence immediately preceding the cycle in progress 

 (9). Unless otherwise stated, it will be understood that subjects in proestrus 

 were 4-day cyclic rats and had just experienced a diestrous interval of 2 days. 

 Subjects said to be in diestrus day 3, on the other hand, were 5-day cyclic 

 rats. Inasmuch as spontaneous "pseudopregnancy" occasionally occurs in 

 this stock, inevitably a few "diestrous" rats turned out to have a leucocytic 

 vaginal smear on the day after experimental treatment. Data from such 

 animals are excluded for present purposes. Unless otherwise specified, all 

 stimulations were confined to the interval between 2.00 and 4.00 p.m. Even 

 on diestrus day 3 it has been shown that sensitivity to progesterone (as an 

 ovulation incitor) is maximal during the afternoon hours (12). Predictability 

 of the hours of the "critical period" is assured by a regime of 14 hr of daily 

 illumination. 



