Rates of photosynthesis were used to evaluate the response of phytoplankton when passed through the cooling system of a power generating facility. Photosynthesis was measured as the uptake of carbon. A factorial design permitted evaluation of heat input, total effect of passage, and degree of recovery. Six experiments measured response at a fixed time of day and at various times in a daily cycle. An increase in temperature of approximately 8.0 C stimulated photosynthesis when natural water tempratures were 16 C or cooler and inhibited photosynthesis when natural water temperatures were 20 C or warmer. Passage through the cooling system resulted in further inhibition when the water was warm and nullified thermal stimulation when the water was cool. No recovery was observed within a four hour period following passage. Photosynthesis in cooled effluent water was either unaffected or actually suppressed. The existence of daily synchrony in the algae was shown by a daily maximum of thermal stimulation at 0900. The rhythm is considered inherent and only amplified by warming.