Abstract. To understand more closely the structural requirements of the LH molecule necessary to stimulate adenylate cyclase, we studied the modulation of this enzyme in partially purified plasma membranes prepared from isolated interstitial cells of rat testis submitted to oLH and to some oLH derivatives and natural analogues. The role of Mg2+ was also investigated in relation to the structural modifications of oLH. Some new facts appeared in this study: 1. Methyl oLH, which exhibited the same ability as native oLH to stimulate cAMP accumulation and steroidogenesis in isolated cells, cannot induce the same level of maximal stimulation of adenylate cyclase as native oLH in plasma membranes. This phenomenon is related to the Mg2+ concentration, and the differences between maximal activation induced by methyl oLH and oLH were more apparent at a free Mg2+ concentration of 3.3 mmol/l than at lower concentrations. The maximal activity (in terms of native oLH) of other alkyl derivatives, such as ethyl or isopropyl oLH, on the contrary, was similar in isolated plasma membranes and in intact cells suggesting that the differential behaviour of the membranes specifically concerns the methyl derivative. 2. Guanidyl oLH and guanidyl porcine LH, which were able to induce cAMP accumulation in intact cells, did not exhibit any stimulating activity in plasma membranes. 3. Among the natural analogues, hCG and pLH are distinguished by a lower maximal activity (by comparison with oLH) particularly at high Mg2+ concentration. This work shows that changes in the LH structure have an impact not only on the parameters of the adenylate cyclase complex but also on the transduction of the hormone signal and its modulation by Mg2+.