The effects of medium glucose concentration (0-20 mmol l-1), pH (7.4 and 6.8) and flow (100 to 33% normal) on lactate uptake and glycolytic flux from 6-3H glucose were studied in perfused livers from 48-h starved rats. At both pH values, the glycolytic flux increased proportionally with the medium glucose concentration. Maximum glycolytic flux at 20 mmol l-1 glucose in the medium was 0.5 mumol min-1 g-1 liver (C6-units) at pH 7.4. At pH 7.4 and 20 mmol l-1 glucose the glycolytic flux decreased approximately proportional with flow. At pH 6.8 the glycolytic flux was extremely low and independent of flow. At flow 33% normal and pH 7.4 a net lactate production was accounted for by glycolysis from medium glucose concentration, indicating virtually no simultaneous lactate uptake. In contrast, at pH 6.8 net lactate production accounted for only half the glycolytic rate, indicating that lactate uptake occurred simultaneously with glycolysis. Thus, glucose-to-lactate flux in liver (as in muscle and brain) is subject to inhibition by low pH, and lactate uptake is enhanced by low pH.