As part of comprehensive 2 by the Metropolitan Water District of Southern California, trihalomethanes were monitored in a major portion of the distribution system under four chlorination schemes. No significant effect of chlorine application point was found. The main objectives76C of the study described were: to monitor the trihalomethane (THM) content in a major portion of Metropolitan's distribution system; to measure the change in THM production under different chlorinating conditions; and to evaluate sampling points and sampling methodology.