Transgenic mice that express human equilibrative nucleoside transporter subtype 1 (hENT1) under the control of a neuron-specific enolase promoter have been generated. Southern blot and PCR revealed the presence of the transgene in five founder mice. Mice from each founder line were examined by reverse transcriptase (RT)-PCR and found to express hENT1 in RNA isolated from whole brain, cerebral cortex, striatum, hippocampus, and cerebellum but not liver, kidney, heart, lung or skeletal muscle. Cortical synaptosomes prepared from transgenic mice had significantly increased [(3)H]adenosine uptake and [(3)H]nitrobenzylthioinosine binding, relative to samples from wild-type mice. In behavioral tests, transgenic mice had altered responses to caffeine and ethanol, two drugs that inhibit and enhance, respectively, adenosine receptor activity. Caffeine-induced locomotor stimulation was attenuated whereas the hypnotic effect of ethanol was enhanced in transgenic mice. Caffeine was more potent in inhibiting ethanol-induced motor incoordination in wild-type than in transgenic mice. No differences in expression of mouse genes for adenosine receptors, nucleoside transporters, or purine metabolizing enzymes were detected by RT-PCR analyses. These data indicate that expression of hENT1 in neurons does not trigger adaptive changes in expression of adenosine-related genes. Instead, hENT1 expression affects dynamic changes in endogenous adenosine levels, as revealed by altered behavioral responses to drugs that affect adenosine receptor signalling.