The results of experimental investigations of heat transfer from a circular pipe to lead coolant with the oxygen content being controlled and monitored are presented. The heat-transfer investigations are conducted for Peclet numbers 800–3550, Prandtl numbers 0.0123–0.0211, and Reynolds numbers 40,000–190,000 with specific heat flux ~40 kW/m 2 and thermodynamically active oxygen content in lead 10-7 –100. The experimental dependences of the Nusselt numers on the Prandtl numbers with different oxygen content in the lead coolant are obtained.