Use by physicians of medical care resources to treat similar patients and morbidities has implications for the efficiency and effectiveness of medical care. This study examined the variation in primary care physicians' use of outpatient laboratory, radiology, drugs, and hospital admissions; the relationships among the uses of these resources by physicians; and uses of these resources by physicians over time. The setting was a large HMO and the physicians were internists (n = 30), pediatricians (n = 20), and obstetrician/gynecologists (n = 17). Resource use was a dollar rate for laboratory, radiology, and drug, and an admission rate per contact for the hospital. The physicians' use rates were derived from the computerized utilization record of a sample of HMO members for 1976-1978. Substantial variation occurred in the rates of use of each resource by physicians in each specialty. The relationships among the resource rates were similar for internists and pediatricians, and were somewhat stronger than expected clinically. There were also substantial consistencies over time in the levels of use of laboratory and drugs by internists and pediatricians. Findings suggest a potential for more judicious use of ambulatory care resources by primary care physicians in this setting.