There is the potential to use continuous sampling whole effluent toxicity monitors, based on respirometry, as early warning systems for water contamination. in order to apply these monitors, however, many issues influencing system response and data analysis must be addressed. This study examines the use of a continuous flow-through toxicity monitor (Bioscan®) to (1) determine the sensitivity, accuracy, and precision of the toxicity monitor to different concentrations of a stressing agent (in this case pH), (2) establish operational and maintenance procedures, and (3) examine the consistency in the measured response to a single perturbation. The system precision was sensitive to all the pH pulse perturbations, with stable background variability (1 % to 6%); however, replicated pulse applications demonstrated that the system has lower accuracy (31 % to 38%). The response consistency was limited, with notable differences in maximum dissolved oxygen and recovery time. The initial response time (inclining slope) was relatively consistent. Although this examination should be repeated with other contaminants used to stress the toxicity monitor, it appears that continuous flow-through toxicity monitors like the Bioscan® are well suited for qualitative early alert warning systems for water contamination.