Three experiments were conducted to evaluate negative feedback action of estradiol-17~ (E2) on luteinizing hormone (LH) release in ovariectomized heifers and to determine the response at different ages. In the first experiment, iv injections of .2 or 2.0/~g E2/kg body weight (BW) reduced serum concentrations of LH by arresting episodic mode of secretion. Results of a second experiment indicated a positive relationship (P<.05) between dose of E2 and duration of LH suppression. Mean duration of suppression with the high (2.0/~g E2/kg BW), medium (2 #g E2/kg BW) and low (.1 /~g E2/kg BW) doses was 6.5, 4.5 and 3.5 h, respectively. Neither magnitude nor rate of LH suppression was related (P>.25) to dose in the first two experiments. Results of third experiment showed that both magnitude and duration of suppression were directly proportional to dose of E2 (0, .1 and 1.0 #g E:/kg BW). The response lasted longer in heifers 4 mo stitutes of Health, for donating luteinizing hormone used for iodination, luteinizing hormone antisera and luteinizing hormone used for standards, and the Upjohn Company for donating estradiol-tyrosine methyl ester. We also thank Dr. R. H. Stauffacher for assistance with surgery; Mr. E. Hoch, Mr. P. Bringie, Mr. J. Kane and Ms. J. Hillstrom for their excellent technical assistance, and Ms. S. Kading for typing this manuscript. 3 Trainee of the Endocrinology-Reproductive Physiology Program. 4 Dept. of Meat and Anita. Sci. and Wisconsin Regional Primate Research Center.of age than in heifers 8 and 12 mo of age. E2 was also more effective in reducing LH pulses in heifers 4 mo of age than in heifers 8 and 12 mo of age. These results show that E2 inhibits LH release by influencing pulsatile mode of secretion, and that the threshold to negative feedback increases as heifers approach puberty. (

Laparoscopy in rhesus monkeys and specialized equipment developed or adapted for this procedure are described. Repeated laparoscopy in the same animal throughout the menstrual cycle showed by morphological and hormonal criteria that this technique does not significantly influence follicular growth, ovulation, luteal function or cycle length. Observations of the side of ovulation and other follicle/corpus luteum relationships are also described.

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