The dose response of a luteinizing hormone releasing hormone (LHRH) agonist in rats with high endogenous gonadotrophin levels was determined. Female rats were ovariectomized and injected with 0.3 microgram (group B), 3.2 micrograms (group C), 32 micrograms (group D) and 320 micrograms (group E) of a slow-releasing microcapsule preparation of the LHRH agonist D-Trp 6-LHRH. Control ovariectomized rats (group A) remained untreated. Plasma luteinizing hormone (LH) concentrations were measured by radioimmunoassay (RIA) before the LHRH agonist injection as well as 5, 15 and 30 days thereafter. Furthermore, LH bioactivity was determined by an in vitro rat LH bioassay in order to evaluate changes in bioactivity after administration of LHRH agonist. In control rats, plasma LH concentrations increased to 4.8 +/- 1.3 ng/ml on day 5, reaching peak levels of 9.9 +/- 1 ng/ml on day 30. In contrast to the control group, those rats which received 320 micrograms of LHRH agonist did not show any increase. Rats which received intermediate doses (groups C and D) tended to maintain levels of LH halfway between group A and group E during the first 15 days of treatment. Thereafter LH concentrations were similar to the untreated control group. The course of the LH concentrations during treatment measured by bioassay (BA) showed a similar pattern to the LH concentrations measured by RIA. The BA/RIA ratio was similar in all groups.