1. Equations of Mass Transport, with Lateral Mixing Taken into Account. The problems of mass transport by ocean currents are no less important than are those of the currents themselves. Recently Sverdrup (1947) and Reid (1948) discussed the inter-relation between mass transport, pressure distribution and vertical mixing; they were successful in showing that the pressure distribution in the Pacific Equatorial Currents is being fairly maintained by the winds. In the present paper it is of some interest to discuss a certain relationship between mass transport and lateral mixing in a rotating enclosed sea.