Pressure responses of a depth-regulatory nature have been demonstrated in all planktonic animals investigated. These animals responded to a rapid increase in pressure by active, continuous or intermittent, upward swimming, while a slow decrease in pressure was followed by less active downward swimming or passive sinking. The transient responses with respect to directional light varied among different species. With illumination directly from above, almost all swam upwards and towards the light in response to an increase in pressure, followed generally by a passive downward sinking or swimming away from the light when the pressure was decreased. With the illumination directly from below, movement of Calanus and Clupea larvae was still directed upwards (and away from the light) after an increase in pressure, while the nauplii of Balanus, Eurydice and zoea larvae of Porcellana remained mostly near or on the bottom, the last two showing evidence of dorsal reflex reactions. In horizontal experiments, with illumination from one side, the responses of most animals to an increase in pressure were first upward and then generally light-ward. With a decrease in pressure, they tended to sink or swim away from the light. Thus the depth-compensatory mechanism relies primarily on orientation to gravity and secondarily, in certain species more than others, on reactions to light. In darkness, however, the responses were clearly oriented to gravity. Sustained pressure increases stimulated the animals to swim upwards, but this was soon followed by a brief temporary depression of activities in megalopa larvae of both Porcellana and Carcinus for no apparent reason.