Functional glycolytic capacity and its regulation have been studied in the fetal guinea-pig heart during O2 deprivation in situ and in the Langendorff perfused heart. Anaerobic glycolytic flux, at 2 mumol/min per g wet wt. was similar in the 48-50 and 60-65 days fetal and adult guinea-pig heart, despite lower fetal phosphofructokinase activity. During O2 deprivation in situ and in the perfused heart glucose was the major substrate, with glycogen making a smaller contribution. Glycolytic capacity became more tightly regulated during fetal heart development. Thus at 48-50 days glycolysis was increased during O2 deprivation by substrate supply, but at 60-65 days activation of phosphofructokinase was required also. Low malate/aspartate cycle activity in the fetal heart was suggested by the absence of an increase in malate and alanine at the expense of aspartate. The large proportion of aerobic glycolytic flux converted to lactate concurred with this. Because of the low O2 consumption and relatively high aerobic glycolytic flux, the proportion of glycolytically-derived ATP was 3-4 fold higher in the fetal than adult heart, and may explain its functional resistance to O2 deprivation.