The origins and behavior of pharmaceutical residuals in the environment is discussed in terms relevant to public water supply. The presentation is based on work conducted as a doctoral project at the University of New South Wales in Sydney, Austraila. The characteristics of the 50 highest use pharmaceuticals by mass in Australia were examined to establish the general characteristics of pharmaceutical residuals as a class. A predictive model was formulated to describe the concentrations of pharmaceutical residuals appearing in raw sewage, their distribution between solid and liquid phases in typical secondary sewage treatment, and the concentrations appearing in solid and liquid effluents as a product of chemical characteristics and degradative processes. Analysis of raw and treated sewage samples from eight sewage treatment plants in Australia, Germany, and the USA was completed to evaluate model characteristics. Results are used to describe the general behavior of organic contaminants in the environment and factors of importance to water supply and contaminant control. Includes 3 references, tables, figures.