Insulin in blood is thought to cross the blood-brain barrier (BBB) to act within the brain to help control appetite. We examined the ability of blood-borne insulin to cross the BBB. Human insulin was infused for 48 h subcutaneously at several doses into mice and the amount of human and murine insulin in serum and brain measured with species-specific radioimmunoassays. For the exogenous human insulin, both brain and blood concentrations increased with increasing doses of infused insulin, whereas for the endogenous murine insulin, brain and blood concentrations decreased. Since the mouse cannot make human insulin, blood was the only source for the human insulin in brain, demonstrating that insulin does indeed cross the BBB. The relationship between the concentrations of human insulin in brain and blood was nonlinear, showing that passage is by a saturable mechanism. Partial saturation of the transporter occurred at euglycemic concentrations of serum insulin. Thus, insulin enters the brain by a saturable transport system that is operational primarily at physiological levels of serum insulin.