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Primed-continuous infusion of [2-3H]- and [U-14C]lactate was used to study the effects of endurance training (running 2 h/day at 29.4 m/min up a 15% gradient) on lactate metabolism in rats. Measurements were made under three metabolic conditions: rest (Re), easy exercise (EE, 13.4 m/min, 1% gradient) and hard exercise (HE, 26.8 m/min, 1% gradient). Blood lactate levels in trained animals increased from 1.0 +/- 0.09 mM in Re to 1.64 +/- 0.21 in EE and 2.66 +/- 0.38 in HE. Control animals also demonstrated an increase in blood lactate with increasing work rate, but values were 1.93 +/- 0.21 and 4.62 +/- 0.57 mM at EE and HE, respectively. Lactate turnover rates (RtLA) measured with [U-14C]lactate increased from 214.0 +/- 17.0 mumol.kg-1.min-1 in Re to 390.3 +/- 31.6 in EE and 518.1 +/- 56.4 in HE. No significant differences in RtLA were observed between controls and trained animals under any condition …