Concentrations of immunoreactive insulin (IRI) and, in some cases, testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured in serum and seminal plasma (SP). IRI levels were usually higher in SP. They were unaffected by masturbation; circulating testosterone, LH, and FSH levels were also unaffected. The IRI concentration was higher in the second (vesicular) fraction of split ejaculates. Semen quality was unrelated to IRI in SP in normal and diabetic (chlorpropamide maintained) men. The addition of insulin in vitro to washed spermatozoa from diabetic and nondiabetic men had no effect on oxygen and glucose uptake or on lactate production and spermatozoal motility. Furthermore, administration of L-arginine to infertile men for 3 months (4 gm/day) was followed in four of seven men by significant increases in IRI concentrations in blood and SP. However, neither semen quality nor fertility was improved.