Mediated by algal symbionts, calcification in reef building corals is one of the important processes, which enable coral's growth. In the present study, we used a buoyant weighing technique to study calcification of two coralline species, Stylophora pistillata and the hydrocoral Millepora dichotoma. The colonies were grown in a tank system, in which light, nutrition and water motion were kept constant and temperature was elevated by means of a computerized controlled apparatus. An almost constant rate of calcification was observed in the two species at 22-28 degrees C. Elevation of the temperature above this range to 29-31 degrees C caused a slow down in calcification in both species. A grater number of S. pistillata colonies became bleached at temperatures of >or=29 degrees C, whereas M. dichotoma colonies suffered from bleaching only after three days at 31 degrees C. For both species, control groups, remained viable during the experimental period. The differences in responses to changes in temperature of the two species may be as a consequence of different adaptive mechanisms or to different susceptibilities of the corals to elevated temperatures. We have shown that elevating temperatures above annual maximal ranges have a significant effect on coral calcification. We also demonstrated that sessile calcified marine organisms having ecological and biomedical significance could be cultured and manipulated under laboratory conditions.