The effect of infusion (2 h) of various sugars (20%, w/v) or early diabetes mellitus (2 h following injection of streptozotocin) on the potential difference (PD) across the brush border of rat jejunum has been studied in vitro. Infusion of glucose or galactose resulted in hyperpolarization of the brush-border PD (-55.2 and -54.9 mV respectively) compared with mannitol-infused animals (-47.7 mV). Infusion of fructose and alpha-methyl glucoside was without effect on PD. Ion substitution experiments showed that the glucose infusion-induced hyperpolarization was due either to a decrease in Cl- conductance and/or increased K+ conductance of the brush-border membrane. Na+ conductance appeared to be unaltered in these experiments. Since galactose infusion hyperpolarized PD without elevating plasma glucose or insulin levels, it seems that neither glucose nor insulin alone is responsible for the significantly greater PD. Treatment with streptozotocin produced glycosuria and hyperglycaemia within approximately 90 min and animals were used 30-60 min later. PD in streptozotocin-treated animals (-51.8 mV) was significantly greater than that measured in diluent-injected animals (-45.3 mV, P < 0.001). However, unlike the situation with sugar infusion, ion substitution experiments showed that the hyperpolarization seen in diabetic animals was due to a decreased Na+ conductance of the brush-border membrane. Since the magnitude of the brush-border PD dictates the electrochemical driving force for Na(+)-sugar cotransport, our data represent the first evidence for an effect of acute hyperglycaemia on the capacity for brush border sugar uptake. However, the enterocyte and systemic factors responsible for the membrane hyperpolarization are unclear.