There is considerable evidence, both from animal studies and clinical reports, to suggest that the hypoglycemic agent phenformin interferes with lactate metabolism. Experimental data are reported which are consistent with a defiCiency in the ability of phenformin‐treated rats to handle endogenous and exogenous lactate. An interesting case of lactic acidosis simulating myocardial infarction is presented and discussed. It is concluded that phenformin can initiate or contribute to the clinical syndrome of lactic acidosis.