Summary. The effects of single or combined daily treatment with an LHRH agonist and low or high doses of LH upon the testes of adult hypophysectomized rats were studied for up to 2 weeks in which changes in testicular histology, particularly the interstitial tissue, were examined by morphometry and related to functional assessment of the Leydig cells in vivo and in vitro. Compared to saline-treated controls, LHRH agonist treatment did not alter testis volume or the composition of the seminiferous epithelium or any of the interstitial tissue components although serum testosterone and in-vitro testosterone production by isolated Leydig cells were significantly reduced. With 2 μg LH for treatment, testis volume was increased, spermatogenesis was qualitatively normal, total Leydig cell volume was increased, serum testosterone values were initially elevated but subsequently declined and in-vitro testosterone production was enhanced. Testis volume with 20 μg LH treatment was unchanged compared to saline treatment, the seminiferous epithelium exhibited severe disruption but total Leydig cell volume was greatly increased due to interstitial cell hyperplasia. This group showed elevated serum testosterone concentrations and major increases in testosterone production in vitro. Treatment with LHRH agonist with either dose of LH resulted in reduced testis volume, moderate to very severe focal spermatogenic disruption and increased total Leydig cell volume although serum testosterone values and in-vitro testosterone production were markedly reduced compared to control rats. It is concluded that, in the absence of the pituitary, LHRH agonist fails to disrupt spermatogenesis and the previously described antitesticular action of LHRH agonists in intact rats is therefore dependent upon the presence of LH, which alone or in combination with LHRH agonist, may focally disrupt spermatogenesis in hypophysectomized rats whereas the Leydig cells undergo hyperplasia. The findings show that impairment of spermatogenesis is accompanied by alterations of the interstitial tissue and suggest that communication between these two compartments is involved in the regulation of testicular function.