Biological effects data with single chemicals are far more abundant than with mixtures. Yet, environmental exposures to chemical mixtures, for example near hazardous waste sites or nonpoint sources, are very common and using test data from single chemicals to approximate effects of mixtures can be useful in environmental risk assessment. To facilitate the linkage, the Weibull function was used as a common model to link responses of single chemicals with the response of their mixtures. The present paper addresses the response of fish to mixtures of narcotic chemicals and a second paper addresses the developmental malformation of frog embryos when exposed to defined mixtures of teratogenic chemicals. Biological effects data with singly tested chemicals cannot be used directly to predict effects of mixtures. However, narcotic chemicals are known to produce an additive concentration effects in fish and the Weibull function with an additive concentration variable was used to model the effects of mixtures of these chemicals. The model produced good agreement with data over a wide range of chemicals and mixture ratios and provides a useful initial assessment of environmental effects of mixtures of narcotic chemicals.