Most chlorine toxicity studies have been conducted under conditions of constant exposure, rather than under intermittent chlorination conditions. The recent emphasis to generate more appropriate data under power plant operational modes will result in a more comprehensive and appropriate basis for criteria development. In the interim, several considerations need to be discussed in detail: (1) toxicity is time, concentration, water quality, and species related; (2) disequilibrium usually results in mortality; (3) various physical, biological, and chemical factors may influence observed effects at plants during chlorination; and (4) a representative research program involves variable exposure times, exposure concentrations, water quality, and both fish and invertebrate species.