This study reports on the relationship between minute ventilation (V E ) and environmental variables of temperature (T) and pressure (P) during open water diving. The author conducted a total of 38 dives involving either a light (20 dives) or a moderate (18 dives) level of physical activity. Within each of these groups, P and T taken together accounted for about two thirds of the variance in the V E data. A very significant increase in V E was observed as T decreased (1 < T( o C) < 22), and the magnitude of this increase at a given pressure level was similar in the 'light' and the 'moderate' data sets. A second order observation, particularly notable at lower temperature, was the decrease in V E with increasing pressure under conditions of light work. Empirical functions of the form V E = A + B n [1 + exp (T - 8)], where A, B, and n are adjustable variables, could accommodate both data sets over the whole range of T and P. These results are the first obtained under actual diving conditions to provide evidence for interactions between P, T, and V E . Understanding the physiological mechanisms by which these interactions occur would assist in appreciation of the limitations imposed on scuba divers by the environmental conditions as they affect their ventilatory responses.