A technique has been devised to accurately inject relatively large quantities of a toxic compound into an experimental aquatic system for toxic stress experiments. This technique has proven superior to the more conventional methods of injecting the toxicant dissolved in a solvent or in an aqueous solution near saturtion. In addition to the large volumetric delivery capability, it was desired to control the injection rate by an on-line monitoring and control computer system. The resulting design resolves the difficulties of: test compounds exhibiting low-solubility in water; highly volatile compounds; rapidly biodegradable compounds; compounds with a high melting point; and accurately preparing and maintaining large quantities of known-concentration solutes.