We examined the hypotheses that 1) smoking acutely before exercise (AS) results in a higher rate of lactate production during exercise compared with chronic smoking with preexercise abstinence (CS) and 2) smokers have a higher rate of lactate conversion to glucose during exercise compared with nonsmokers (NS). To test our hypotheses, seven male smokers and seven nonsmokers were studied by using a primed continuous infusion of [3-13C]lactate during 90 min of rest and 60 min of exercise on a cycle ergometer at 50% peak O2 consumption; smokers were studied twice: once after an overnight smoking abstinence and once after smoking three cigarettes before exercise. The rates of lactate appearance and conversion to glucose were increased markedly with exercise compared with rest in all groups (P < 0.05); the rate of lactate appearance for AS was significantly greater (7.87 +/- 0.77 mg.kg-1.min-1) than for both CS (4.64 +/- 0.33 mg.kg-1.min-1) and NS (5.57 +/- 0.60 mg.kg-1.min-1) (P < 0.05). The rate of lactate conversion to glucose was similar between CS and AS (6.49 +/- 1.82 and 6.30 +/- 1.69 mg.kg-1.min-1, respectively) during exercise; NS had a significantly lower rate (3.31 +/- 0.90 mg.kg-1.min-1) compared with CS and AS (P < 0.05). In summary, acute smoking increases lactate flux during exercise; in addition, smokers have a higher rate of lactate to glucose conversion during exercise compared with nonsmokers, which may indicate an increased glucose dependency.