Neutron activation analysis has been made on U and Th in possible mantle materials, mainly peridotite nodules from continental and oceanic areas. It seems that oceanic peridotite nodules have more uniform contents of U and Th, and Th/U ratio, than continental ones. Average contents of 8 oceanic lherzolites are (2.14 ± 0.26) × 10 −8 g/g U and (5.48 ± 0.87) × 10 −8 g/g Th. If the terrestrial ratio is assumed for K/U, the heat production rate of these lherzolites is about 80% of that of chondrites. This value is too small to account for the observed heat flow of the average oceanic upper mantle.