Few studies have systematically examined the influence of physician, patient, and practice characteristics on physician-directed use of resources within the overall environment of medical group practices and none have included the practice culture in the analysis. This study analyzes the effects of the structure and culture of medical group practices on the amount of resources used to manage uncomplicated hypertension episodes of care for enrollees in a Minneapolis/St. Paul HMO during 1990. Three findings emerged from this study: (1) resource use for a well-defined episode of care varies much more than one would expect in this highly competitive managed care environment; (2) the culture of the group practice appears to be more important than organizational structure in determining resource use for the treatment of hypertension; and (3) together the culture and structural variables only explain 8 percent of the variance in resource use. The study indicated that medical group practice organizations have less influence on physicians' practice styles than expected. The group practices studied are all located in a highly competitive managed care environment and these conditions should be causing them to create more standardized practice styles among their physicians. However, wide variations in individual physician practice styles account for most of the differences observed. Either much of the unexplained variance in resource use for this episode of care results from unobserved patient and illness characteristics, or managed healthcare is not yet causing medical group practices in Minnesota to challenge physicians' individualistic practice styles.