The aim of this study was to obtain in rainbow trout evidence for the role of lactate in liver carbohydrate metabolism. In the first experiment fish were injected intraperitoneally (n=8) with 5 mL x kg(-1) of Cortland saline alone (control) or saline containing L-(+)-lactate (22.5 mg x kg(-1) or 45 mg x kg(-1)) with samples being obtained 6 h after treatment. In the second experiment, to isolate the effects of increased lactate levels alone from the possible in vivo interaction of increased lactate levels with the effect of hormones and metabolites other than glucose, small liver pieces were incubated in vitro for 1 h at 15 degrees C in modified Hanks' medium containing 2, 4 or 8 mM L-(+)-lactate alone (control) or with 50 mM oxamate, 1 mM DIDS, 1 mM dichloroacetate (DCA), 10 mM 2-deoxyglucose (2-DG), 1 mM alpha-cyano 4-hydroxy cinnamate (4-CIN) or 10 mM D-glucose. The response of parameters assessed (metabolite levels and enzyme activities) provided evidence for some characteristics of lactate metabolism in fish liver that were not present when specific inhibitors were used. The main in vivo effects of lactate treatment were increased levels of lactate (approx. 100% increase) and glucose (30-70%) in plasma, as well as decreased glycogen (50%) and lactate (30%) levels, and increased gluconeogenic (20%) and glycolytic (50%) potentials in liver. Those actions, however, were probably the result of an indirect action with other substrates (glucose) and/or hormones since in vitro experiments did not provide similar results for those parameters.