Several structure-activity relationships have been published for estimating the lethality of nonpolar nonelectrolytes to fish. The vast majority of non-reactive industrial chemicals produce toxicity symptoms consistent with narcosis. However, researchers have found that many chemicals which appear to produce narcosis, are substantially more toxic than the published structure-toxicity relationship predicts. Researchers observed that these chemicals are more polar and often have acidic hydrogen bond donor functional groups. The data are consistent with the 'polar' narcotic class proposed by Ferguson five decades ago. The structure-toxicity relationship for polar narcotics (n=39; r2 = 0.90) with fathead minnows (Pimephales promelas) is: log LC50 = -0.65 (+ or - 0.07) log P-2.29 (+ or - 0.22). The paper presents the data for the relationship and discusses the structural requirements for the toxicity syndrome.