The effects of l-arginine and luteinizing hormone (LH) on testosterone production by rat Leydig cells. Purified rat Leydig cells were prepared as described in “Materials and Methods” and were incubated in l-arginine–free Leydig cell medium (LCMA), LCMA with added l-arginine (5 mM, LCMA + Arg), LCMA in the presence of LH (0.5 ng/mL, LCMA + LH), and LCMA in the presence of LH and l-arginine (LCMA + LH + Arg). Each bar represents the mean ± SEM for testosterone production and is the outcome of 3 separate determinations repeated with similar results.