Quantitative structure–activity relationship (QSAR) estimates of toxicity of narcotic chemicals for 19 species of bacteria, algae, fungi, protozoans, coelenterates, rotifers, molluscs, crustaceans, insects, fish, and amphibians were used to predict no-effect levels (NELs) at the ecosystem level by means of recently developed extrapolation methods. Equilibrium partitioning theory was used to derive NELs for aquatic sediments and internal toxicant concentrations for aquatic organisms. A simple table is given from which NELs for narcotic chemicals for water, sediment, and residues in biota can be predicted on the basis of only the octanol/water partition coefficient and molecular weight. The method may be applied to setting quality criteria for the aquatic environment and to ecotoxicological interpretation of (bio)monitoring data. Calculations were carried out for 102 narcotic compounds.