Plasma concentrations of [met]enkephalin (ME) and beta-endorphin (beta E) were measured in samples obtained immediately before and after physiotherapeutic exercises for patients with ankylosing spondylitis (AS), osteoarthritis (OA), or knee injuries. Correlations were sought between opioid peptide concentrations or changes therein, and nature, severity and duration of disease, age, severity of pain reported and pain threshold. No correlation was found with any of the pain parameters. However, there was a possible relationship between age or duration of disease and changes in ME concentrations.