A microprocessor- controlled coulometric system for measuring total inorganic carbon in seawater samples was refined. A precision of 0.5 micromoles/kg (one standard error) from a single sample was routinely achieved in the laboratory. This was reduced to 1.0 micromoles/kg at sea due to an increase in the instrument blank. The system has proved to be a robust and reliable method, well suited for routine plankton productivity measurements and oceanic mapping. The coulometric technique was used in conjunction with an automated Winkler oxygen method to make in vitro and in situ observations of planktonic photosynthetic ana respiratory quotients. In common wi th earlier published observations, a wide range of quotients were obtained. A detailed consideration of analytical and sampling errors led to the conclusion that the quotients could mostly be explained by the stoichiometry of conventional biochemical products and reactants. In situ observations were carried out within mesoCosms:-Tn one field study, particular attention was paid to the correction of data for physical mixing within the mesocosm and exchange with the atmosphere. A dye diffusion study coupled wi th continuous temperature profiles led to the the conclusion tha t the circulation wi thin these bags is complex and cannot be described by use of a one-dimensional vertical diffusion model. The total inorganic carbon measurements determined from vertical CTD profiles during the 1987 RV Challenger cruise were compared to similar stations sampled during the Transient Tracers in the Ocean Programme (1981). Although the TTO measurements were calculated from a potentiometric titra tion there was no evidence for major errors of accuracy.