In animals, dichloroacetate (DCA) which activates pyruvate dehydrogenase has been shown to diminish increased blood lactate concentrations due to biguanide treatment. In 10 maturity onset diabetics, therefore, the effect of a combined therapy with buformin and DCA (200 mg b.i.d.) was studied on blood lactate concentrations and compared with an analogous pre- and postinvestigation period of 6 days with buformin treatment alone (100 mg b.i.d.). Mean blood glucose concentrations remained the same during all 3 investigation periods. Also, neither fasting nor postprandially significant differences were found in blood lactate and ketones. In association with a standardized ergometer test, however, the rise in blood lactate was significantly smaller (p less than 0.05) while the patients were on buformin plus DCA, compared to the periods when only buformin was given. Furthermore, less ketone bodies appeared to be utilized by the exercising muscle under the influence of the combined treatment (p less than 0.05). These results are in good agreement with animal studies and suggest that DCA might be as effective in decreasing enhanced blood lactate concentrations in biguanide treated man as in animals.