In mammals, exercise is known to stimulate glycolysis, lactate production and lactate disposal. Monocarboxylate transporters (MCTs) mediate inter-tissue lactate trafficking and orchestrate recovery. By contrast, the effects of swimming on lactate kinetics and the existence of MCTs have never been characterized for rainbow trout (Oncorhynchus mykiss), an essential animal model in ectotherm physiology. Therefore, our goals were: (1) to determine the rates of lactate appearance (Ra) and disposal (Rd) during steady-state swimming at 1.7 body lengths/s (BL/s) and graded swimming at 0.5-2.8 BL/s by in vivo continuous infusion of [U-14C] lactate as described in (Omlin and Weber, 2010); (2) to clone trout MCTs, and to determine their tissue distribution and the effects of exhausting exercise on their level of mRNA expression. We reasoned that such measurements would prove important to understand the mechanisms underlying the classic “lactate retention” of trout white muscle after maximal exercise. Results show that steady-state exercise increased Ra and Rd lactate in parallel from resting values of 23 µmol kg-1 min-1 to maximal levels of 29 µmol kg-1 min-1, maintaining plasma lactate concentration below 1.5 mM. For graded exercise, lactate fluxes stayed at baseline below 2.2 BL/s, but both Ra and Rd lactate were stimulated at higher speeds, reaching maximal values of 40 and 35 µmol kg-1 min-1, respectively. Plasma lactate reached 5 mM at high swimming speeds because the increase in lactate use was insufficient to match the increase in lactate production. We also show for the first time that rainbow trout possess MCTs by cloning MCT1a, MCT1b, and MCT2. These lactate transporters are present ubiquitously, but their expression varies greatly between tissues and isoforms. Exhausting exercise increased the expression of MCT1a (+90%) and MCT1b (+50%) in the heart, as well as the expression of MCT2 in the brain (+260%). Other tissues showed no changes after exercise, and we could not clone MCT4, the isoform mainly dedicated to lactate efflux from glycolytic muscles in mammals. The stimulation of MCT expression in heart and brain is consistent with an accelerated use of lactate by these oxidative tissues, and with the observed increase in Rd lactate. Results also provide a possible functional explanation for lactate retention because: (1) white muscle shows the lowest MCT expression of all trout tissues, (2) white muscle MCTs are not stimulated by exercise, and (3) rainbow trout may not express MCT4. All procedures were approved by the Animal Care Committee of the University of Ottawa. This research was supported by an NSERC discovery grant.