Absence of a diurnal rhythm in lordosis behaviour induced by oestrogen in gonadectomized rats

Gonadectomized rats bearing s.c. Silastic capsules containing crystalline oestradiol-17 beta diluted with cholesterol, or oestradiol-17 beta dissolved in sesame oil were tested for the presence of a diurnal rhythm in the display of lordotic behaviour. In experiment 1, female rats received four consecutive tests at intervals of 8 h in a lighting regimen of 12 h light: 12 h darkness beginning 4, 14 and 28 days after implantation of 5 mm capsules of oestradiol. After a single test on day 4, male rats were tested on days 14-15 only, at the same times as the female rats. Female animals were tested while vaginal-cervical stimulation was prevented by vaginal masking beginning 35 days after implantation of oestradiol. In experiment 2, lordotic responsiveness of female rats was assessed beginning 4 days after implantation of oestradiol once on each of 3 consecutive days, with each test occurring at a different time of day. Finally, in experiment 3, female rats were tested as in experiment 1 beginning 4 dys after implantation of lower threshold amounts of oestradiol in oil-filled capsules. In no experiment were changes in lordotic behaviour observed as a function of the time of day. These findings failed to support recent reports of a sexually dimorphic rhythm in lordotic responsiveness to oestradiol in the rat.