Following previous observations of the potent antifertility effects of acute treatment with LHRH or its agonistic analogs in male and female rats, this study describes the effect of long term (up to 12 weeks) treatment with an LHRH agonist, [D-Ala6, des-Gly-NH210]LHRH ethylamide (LHRH-A), on testicular and ovarian gonadotropin receptor levels and function in the rat. Treatment of adult male rats with 100 ng LHRH-A every third day led to a progressive decrease of testis, seminal vesicle, and prostate weight up to 12 weeks of treatment, while the inhibitory effect (70%) on LH receptor levels was already maximal at 1 week. When adult female rats were injected daily with 5 microgram LHRH-A for 12 weeks, ovarian LH, FSH, and PRL receptor levels were reduced 50--90%, while plasma estradiol and progesterone levels were inhibited 60% (compared with diestrus day 1). The effect of chronic administration of a low dose (5 IU) of hCG in the male rat was transient. By contrast, no sign of resistance developed up to 12 weeks of treatment with the LHRH agonist in either male or female animals.