In eight patients a sparteine-test was carried out immediately before and after 1 week of treatment with quinidine 600-800 mg day-1. Before treatment one patient was classified as a poor metaboliser (metabolic ratio: greater than or equal to 20), and seven patients as extensive metabolisers. During quinidine treatment, the formation of sparteine metabolites (2- and 5-dehydrosparteine) was practically abolished. Patients initially classified as extensive metabolisers thus exhibited the phenotype of poor metabolisers during quinidine treatment.