Exhalative sediments are formed on a large scale on spreading (or active) ridges. Such sediments on the crest of the East Pacific Rise (EPR) are on the average rich in U (38 ppm) and poor in Th (2.4 ppm), contrary to other pelagic sediments that contan 3-6 ppm U and 3-11 ppm Th in the non-biogenic fraction. Most U in active ridge deposits is absorbed from sea water but some derives from different volcanic sources: (a) Shallow hydrothermal leaching of basalts form solutions with high SUP-234 U/SUP-238 U values; (b) Deep seated sources deliver solutions which are very rich in U but with radiochemical equilibrium between SUP-234 U and SUP-238 U; (c) Hydrothermal leaching of sediments is possibly an additional process. Bottom waters over the EPR show normal sea-water values for SUP-234 U/SUP-238 U, suggesting that volcanic U is delivered intemittently. Terrigenous matter delivers most of the Th, whereas biological matter probably is of negligible importance on active ridges as a source for Th. Volcanic processes are negligible sources of U in deposits formed far from spreading centers, such as the bauer Deep sediments and manganese nodules, since any adsorbed U is redissolved during diagenesis. Metamorphic alterations of U rich exhalative deposits could give them the reappearance of vein deposits; a detailed study of such deposits will probably reveal that many of them originally were sedimentary deposits.