Prescriptions issued by 14 general practitioners during December 1974 were examined to elicit possible determinants of differences in doctors' prescribing costs per registered patient. Higher costs were associated with a generally increased prescribing rate but differences were particularly marked for certain drug groups (such as anti-rheumatic drugs). Costs were not related to list size, size of practice unit, urbanization, proportion of elderly patients, or date of qualification of the doctor, and the cost differences were too large to be accounted for by differences in morbidity. The frequency of prescription of drugs considered to be undesirable was also not associated with level of costs.An index of quality of prescribing was constructed. No relationship was found between scores on this index and any of the practice variables studied, nor was there any relationship with high-cost prescribers.