Water quality criteria and standards are based primarily on toxicity tests carried out with single chemicals whose concentration is as nearly constant as possible. In the 'real world', however, organisms are exposed to mixtures of chemicals which usually have markedly fluctuating concentrations. The primary difficulties in simulating 'real world' conditions in toxicity testing are: (a) a means of varying chemical concentration to fit a predetermined set of conditions, and (b) a system which is capable of tracking and recording the response of aquatic organisms to these variations and which is quantitative and suitable for cross correlations of dose and response. Mini- and microcomputer interfacing with a toxicity testing system provides a means of systematically varying the concentration of a test chemical or chemicals in a continuous flow system. The same computer can also be used for the data acquisition system to store the voluminous time-series biological response data necessary for cross correlations with variable chemical concentrations. A description of the apparatus, examples of its use, types of data generated, and data analysis are discussed.