To investigate the mechanism whereby changes in plasma glucose level alter human pancreatic A-cell activity in vivo, A-cell activity was determined during manipulation of plasma glucose and pancreatic B-cell activity by insulin and glucose infusions. A-cell activity (the acute immunoreactive glucagon response to intravenous arginine, 0-10 min) rose from 482 +/- 125 to 968 +/- 191 pg X ml-1 X 10 min-1 (mean +/- SEM) when the plasma C-peptide level (a measure of B-cell activity) was suppressed from 2164 +/- 365 to 872 +/- 162 pg/ml by an insulin infusion at euglycaemia (employing the glucose clamp technique) in six normal subjects. Raising plasma glucose to 6.7 mmol/l during the same insulin infusion returned mean C-peptide (2688 +/- 581 pg/ml) and the acute glucagon response to arginine (447 +/- 146 pg X ml-1 X 10 min-1) close to basal levels. Individual changes in the acute glucagon response to arginine followed the C-peptide changes. The mean change in the acute glucagon response to arginine per unit change in plasma glucose (-191 +/- 36) was similar to that seen when plasma glucose was raised to twice basal levels in six different subjects without an insulin infusion (-159 +/- 45). This suggests that, when plasma glucose is raised to about twice basal level in vivo, the major factor in suppressing A-cell activity is the concurrent change in B-cell activity rather than direct effects of glucose or circulating insulin on the A cell.