Using a Leydig tumor cell line (MA-10) transiently transfected with the human lutropin receptor (hLHR) and mutants thereof, we examined the identity of the G proteins activated by the agonist-engaged hLHR-wild type (wt) and by three of its naturally occurring constitutively active mutants. Two of the mutants examined, L457R in transmembrane helix 3 and D578Y in transmembrane helix 6, are germ-line mutations found in boys with Leydig cell hyperplasia and precocious puberty. The third, D578H, is a somatic mutation found in Leydig cell tumors in boys with precocious puberty. We show that the hLHR-wt and the three mutants activate the G(s), G(i/o), and G(q/11), but not the G(12/13), families of G proteins. The activation of these G proteins by the hLHR-wt occurs only when engaged by agonist, but their activation by the L457R, D578Y, and D578H mutants occurs independently of agonist stimulation. We conclude that the G proteins activated by constitutively active mutants of the hLHR associated with Leydig cell hyperplasia or tumors are identical and are the same as those activated by the agonist-engaged hLHR-wt. If there was preferential activation of some G protein families by the somatic D578H mutation found in Leydig cell tumors as opposed to the germ line mutations found in Leydig cell hyperplasia, then one could envision mechanisms by which the D578H mutant would be oncogenic. The data presented here suggest that such mechanisms do not need to be considered.