Rainbow trout (Salmo gairdneri) were force fed, with and without M.S. 222 anesthesia, pellets containing DDT equivalent to 0, 10, and 100% of the 96 h-LD50 dose of 0.03 mg DDT/g body weight. DDT at the LD50 dose significantly increased learning rate 48 and 72 h after treatment. Although the 10% LD50 group did not learn significantly faster than controls, their slightly improved learning rate indicated a dose-dependent response curve. M.S. 222 did not affect learning rate of a simple conditioned response whether or not the anesthetic was combined with DDT.