This study examines the assumption that the dimensional structure of mood following strenuous physical exercise is unchanged by comparison with that beforehand. A specially-devised adjective checklist was completed by 165 regular runners 5-10 minutes before, and immediately after, a 3-mile run. Principal components analyses of the scores revealed independent dimensions of positive and negative affect which were similar on both occasions. There was no evidence of a separate major dimension of elation or well-being after running. The structure of mood differed before and after running only in minor components. Component-based scale scores were calculated for each subject, for each occasion. Positive mood was increased, and negative mood decreased, after running. The main changes in mood caused by running in these conditions are therefore quantitative rather than qualitative. Improvements in mood were greater in women than in men, largely because women experienced a worse mood state than did men before running.