Pieces of coral from two species, Montastrea annularis and Acropora cervicornis, were exposed in the laboratory to concentrations of 0, 1, 10, and 100 ppm drilling mud for periods of two days to seven weeks. Several physiological functions of the coral animal (calcification rate, respiration rate) and of their zooxanthellae (photosynthesis rate, nutrient uptake rate) were monitored at regular intervals during the exposure periods. In addition, biomass parameters (tissue nitrogen, zooxanthellae cell density, chlorophyll content) were measured at two-week intervals for the longer exposure experiment, and at the end of each experiment for the shorter exposures. Initial long-term exposures of pieces of Montastrea annularis to a series of drill muds (designated JX-2 through JX-7) collected from a Jay oil-field well showed a significant detrimental effect on calcification, respiration, and NO3 uptake rates during the fourth week of exposure to 100 ppm drill mud. Photosynthesis and NH4 uptake rates were affected also during the fifth week of exposure. Normal feeding behavior was absent from these corals when tested during the sixth and seventh weeks of exposure. Several 100 ppm corals died during the fifth and sixth weeks.

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