To examine the effect of increased gluconeogenesis [by increasing free fatty acids (FFA)] on hepatic glucose output (HGO) and on the first substrate (glucose) cycle, a primed continuous infusion of [2-3H]- and [6-14C]glucose was infused to isotopic steady state in 12 normal male volunteers after an overnight fast. Blood samples for the determination of glucose specific activity were obtained before and after an infusion of saline (n = 6) or 10% Intralipid and heparin (90 mU.kg-1.min-1, n = 6). Plasma FFA (593.3 +/- 74.5 to 971.1 +/- 127.1 mumol/l, P = 0.007) and glycerol (68.0 +/- 5.9 vs. 222.4 +/- 32.0 mumol/l, P = 0.002) increased during the lipid infusion, and beta-hydroxybutyrate levels rose from 0.24 +/- 0.12 to 0.50 +/- 0.17 mmol/l (P = 0.01). No change in plasma glucose, insulin, or glucagon levels was observed during the study, and levels of the gluconeogenic substrates alanine and lactate were also unchanged. Baseline rates of glucose cycling (rate of appearance of [2-3H]glucose minus rate of appearance of [6-14C]glucose) were similar in the two groups [1.44 +/- 0.33 vs. 1.33 +/- 0.44 mumol.kg-1.min-1, not significant (NS)] and did not change during either saline or lipid infusion, respectively. However, Cori cycle activity (the conversion of [6-14C]- to [1-14C]glucose) increased significantly from 0.59 +/- 0.19 to 1.28 +/- 0.19 mumol.kg-1.min-1 (P = 0.002) after FFA and glycerol levels had been increased, in marked contrast to the saline control (0.51 +/- 0.18 to 0.39 +/- 0.18 mumol.kg-1.min-1, NS).(ABSTRACT TRUNCATED AT 250 WORDS)