The sensitivity of protozoa, and particularly ciliated protozoa, to environmental changes suggested a study on the physiological responses arising from exposure to toxic compounds. Tetrahymena pyriformis was used as a test organism in a set of miniaturized assays. The physiological response of this ciliate was assessed in terms of adenosine-5'-triphosphate content and acid phosphatase activity after exposure of the cultures of T. pyriformis to four toxicants: copper, zinc, Triton X-100, and cycloheximide. In the range of concentrations used, stimulation and inhibition of these two parameters were observed. The correlation between the two parameters is analyzed.