The interrelationship between glucagon action on splanchnic glucose output and cyclic AMP production was studied in healthy volunteers after hepatic venous catheterization. Glucagon was infused according to four different protocols to achieve arterial levels ranging from 300 to 9000 ng/l. Infusion of glucagon which resulted in arterial levels of the hormone of 4000-9000 ng/l was associated with a marked increase in net splanchnic cyclic AMP production and in the arterial levels of the cyclic nucleotide. The rise in cyclic AMP efflux from the splanchnic area was transient but an augmented splanchnic production was still evident after 30 min of glucagon infusion. Splanchnic glucose output rose 3-5 fold. Infusion of glucagon at lower rates, resulting in arterial levels of 300-900 ng/l, did not measureably stimulate the efflux of cyclic AMP from the splanchnic area. In spite of this, splanchnic glucose output rose 2-3 fold and the blood glucose level increased 20-50% during glucagon infusion at these lower rates. It is concluded that (1) factors other than cyclic AMP are rate limiting in the stimulation of hepatic glucose production, and (2) although cyclic AMP is an established 'second messenger' of glucagon action, other factors may also be of importance in mediating the physiological response of this hormone.