Experimental measurements of the sinking velocity of anaesthetized planktonic animals carried out in the first half of this century prove that the downward migration of planktonic populations may be entirely attributed to passive sinking. This conclusion, along with data from voluminous literature devoted to circadian rhythms, supports the supposition that dM vertical migration is the result of an innate alternation of phases of high and low activity. During the phase of low locomotory activity, a passive sinking of an organism of negative buoyancy takes place; during the phase of high activity, the same organism actively climbs to the surface using the well-known mechanisms of space orientation. This hypothesis offers an explanation both of the usual course of dM migration and various deviations from this course, some of which are mentioned here. If the above considerations are correct, and ff we admit that one of the trends of the evolution of pelagic organisms is directed towards the reduction of energy expenditure for the maintenance of the body in a suspended state, i.e. towards neutral buoyancy, the conclusion may be drawn that dM vertical migrations are not the result of adaptation to the planktonic mode of life.