Increasing doses (15 to 1000 micrograms/ml) of 6-hydroxydopamine (6-OHDA) stimulated, then suppressed testosterone production by mouse leydig cells incubated for 3 h in vitro. The stimulating doses ranged between 50 and 100 micrograms/ml, with maximal effects occurring at 30 to 60 minutes after the start of the treatments. At doses of 500 micrograms/ml, 6-OHDA exhibited inhibitory effects. When added to leydig cell incubations together with stimulating doses of luteinising hormone (LH), 1-(3',4'-dihydroxyphenil)2-isopropylaminoethanol (L-isoproterenol) or 8-bromoadenosine 3':5'-cyclic monophosphate (8-Br-cAMP), 6-OHDA abolished the effects of the latter compounds. Prolactin and prostaglandin E2 (PGE2) inhibited the stimulating effects of 8-Br-cAMP but not LH. It is proposed that the actions of 8-OHDA affect intracellular sites yet to be identified, thereby inhibiting testosterone production by mouse Leydig cells. Some of the actions of 6-OHDA seem to be medicated via beta-adrenergic receptors as the latter abolishes the stimulatory effects of L-isoproterenol, a potent beta-adrenergic agonist. However, the inability of stimulatory doses of LH and 8-Br-cAMP to reverse the inhibitory effects of 6-OHDA point to the possibility that other actions of 6-OHDA may be relayed via a second messenger system separate from that involving cAMP.