An injector is described for delivering a series of constant concentrations of toxicant in a flow-through toxicity test. The apparatus can be employed either as a single channel unit for use with a proportional diluter or as a multichannel injector for use with an equal-volume water delivery system. The multichannel approach is emphasized for pesticide addition because the test concentrations are independent of each other. This allows the establishment of an unlimited range of toxicant concentrations.

Water is passed through a series of buckets which are capable of downward vertical displacement. The dropping buckets operate a series of pipettes which introduce the toxicant material to the system. Each channel is calibrated independently by controlling the stroke distance of the pipette plunger. A return spring mechanism automatically reprimes the pipettes for the subsequent water cycle.

Although the system is suitable for toxicity tests employing freshwater diluent, the metal components of the injector may be susceptible to corrosion under saltwater conditions.