The aim of this study was to obtain evidence in rainbow trout for a role of lactate in glucose homeostasis as well as in the function of glucosensing tissues. In a first set of experiments, trout were injected, either (1) intraperitoneally (N=8) with 5 ml kg(-1) of Cortland saline alone (control) or saline containing l-(+)-lactate (22.5 mg kg(-1) or 45 mg kg(-1)), oxamate (22.5 mg kg(-1)) or d-glucose (500 mg kg(-1)), or (2) intracerebroventricularly (N=11) with 1 microl 100 g(-1) body mass of Cortland saline alone (control) or containing d-glucose (400 microg microl(-1)) or l-(+)-lactate (400 microg microl(-1)), with samples being obtained 6 h after treatment. In a second set of experiments, hypothalamus, hindbrain and Brockmann bodies were incubated in vitro for 1 h at 15 degrees C in modified Hanks' medium containing 2, 4 or 8 mmol l(-1) l-(+)-lactate alone (control) or with 50 mmol l(-1) oxamate, 1 mmol l(-1) DIDS, 1 mmol l(-1) dichloroacetate, 10 mmol l(-1) 2-deoxy-d-glucose, 1 mmol l(-1) alpha-cyano-4-hydroxy cinnamate or 10 mmol l(-1) d-glucose. The response of parameters assessed (metabolite levels, enzyme activities and glucokinase expression) in tissues provided evidence for (1) a role for lactate in the regulation of glucose homeostasis through changes not only in brain regions but also in liver energy metabolism, which are further reflected in changes in plasma levels of metabolites; (2) the possible presence in trout brain of an astrocyte-neuron lactate shuttle similar to that found in mammals; and (3) the lack of capacity of lactate to mimic in vitro (but not in vivo) glucose effects in fish glucosensing regions.