Bilateral lesions of the ventromedial nucleus(VMN) were performed in the 3rd cycle of a sequence of 4-day cycles in Wistar rats by means of stereotaxically-oriented platinum unipolar electrodes. Small (SVMN) or large (LVMN) lesions were produced by the passage of a cathodic current for 7 or 14 sec, respectively. Groups of intact 4-day cyclic and 5-day cyclic rats, and of dorsomedial nucleus (DMN) lesioned and VMN sham (SH) operated females served as controls. A modification of the duration of one or both of the 2 first post-operative cycles was found in 31 out of 60 SVMN and in 11 out of 12 LVMN lesioned females. Only 2 out of 16 DMN lesioned and none of the 17 SH operated rats exhibited a change of post-operative cycle length. No modification of the duration of the 4th and/or the 5th cycles occurred in control non-operated females after a sequence of three 4-day cycles. Further study of vaginal cycles in animals with modified cycle duration showed that estrous rhythm changed from 4- to 5-day or to alternate 4- and 5-day cycles following VMN lesions. Ovulation regularly occurred during the night following proestrus of either 4- or 5-day cycles in VMN lesioned animals. Neither frequency of ovulation nor mean number of corpora lutea/rat differed when compared with DMN lesioned or SH operated females. On the contrary, a significant diminution of sexual receptivity resulted from SVMN lesions in comparison with DMN and SH animals (4-day cycles) or with intact control animals (5-day cycles). LVMN lesions almost completely abolished mating receptivity.

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