This study investigates the physiological responses in the hermatypic coral Porites lutea when exposed to a combination of reduced salinity (from ambient 30 psu to 20 psu) and two concentrations of copper (CuS04), 10 microg 1(-1) and 30 microg 1(-1). Corals were exposed for 14 h and changes in metabolism in terms of primary production rate per chlorophyll a and respiration per surface area (cm2) were used as measures of stress. The results showed no significant changes in respiration rate in any of the treatments compared with controls, or between treatments. The primary production rate, however, displayed a more complex pattern. Corals exposed to reduced salinity, 30 microg 1(-1) copper, and the combination of the two stressors significantly reduced the production rate, whereas corals exposed to 10 microg 1(-1) only, remained unaffected. However, adding 10 microg 1(-1) copper to reduced salinity did not affect the production rate thus indicating an antagonistic effect.