The rate of hepatic glucose production (R(a)glucose) was measured by continuous infusions of 6-[(3)H]glucose in live rainbow trout (Oncorhynchus mykiss) before, during and after swimming for 3 h at 1.5 body lengths s(-)(1) in a swim tunnel. Contrary to expectation, we found that sustained swimming causes a 33 % decline in the R(a),(glucose) of trout (from 7.6+/-2.1 to 5.1+/-1.3 (&mgr;)mol kg(-)(1 )min(-)(1), means +/- s.e.m., N=7), even though exercise of the same intensity elicits a two- to fourfold increase in all the mammalian species investigated to date. Measurements of catecholamine levels show that circulating [epinephrine] decreases by 30 % during exercise (from 4.7+/-0.3 to 3.3+/-0.4 nmol l(-)(1), N=8), suggesting that this hormone is partly responsible for controlling the decline in R(a)glucose. The inhibiting effect of swimming on hepatic glucose production persists for at least 1 h after the cessation of exercise. In addition, rainbow trout can maintain a steady blood glucose concentration throughout sustained exercise by closely matching hepatic glucose production with peripheral glucose utilization, even though this species is generally considered to be a poor glucoregulator. This study provides the first continuous measurements of glucose kinetics during the transition from rest to work in an ectotherm and it suggests that circulating glucose is not an important fuel for aerobic locomotion in trout.