This study examines how the volume of privately insured services provided in hospital inpatient and outpatient departments changes in response to reductions in Medicare physician payments. We hypothesize that physicians consider relative payment rates when choosing which patients to treat in their practices. When Medicare reduces its payments for surgical procedures, as it did in the late 1980s, physicians are predicted to treat more privately insured patients because they become more lucrative. We use data from 182 hospitals for seventeen major procedures groups, covering a forty-five-month period between 1988 and 1991 that encompasses a twenty-four-month period before the reduction in Medicare fees and twenty-one months after the reduction. Our findings are consistent with the predictions for a number of procedure groups, but not for all of them. One implication of the findings is that societal savings from Medicare fee reductions are overstated if one does not also consider spillover effects in the private insurance market.