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Two projects have been carried out to study the application of human operator describing functions in the investigation of the influence of drugs (alcohol and marijuana) on subjects performing a compensatory visual-manual tracking task. The describing functions were measured through the application of power spectral density techniques to the signals circulating in the control loop. A range of drug dosages was employed which included alcohol alone, marijuana alone, and a combined dose of marijuana and alcohol. Significant alterations in the describing functions were observed and interpreted as changes in amplitude, phase, and operator injected noise (remnant). Linear models fitted to the raw describing function data are used to summarize the observed trends in the human operator's dynamic characteristics. The results provide an insight into the changes in human performance produced by alcohol and marijuana.