Caffeine is believed to exert its stimulant effects by blocking A_2A and A₁ adenosine receptors (A_2AR and A₁R). Although a genetic knockout of A_2AR eliminates effects of caffeine, the phenotype of the knockout animal does not resemble that of caffeine treatment. Here we explored the possibility that a mere reduction of the number of A₁Rs and A_2ARs, achieved by deleting one of the two copies of the A₁R and A_2AR genes would mimic some aspects of long-term caffeine ingestion. The A₁R and A_2AR double heterozygous (A₁R-A_2AR dHz) mice indeed had approximately half the number of A₁R and A_2AR, and there were little compensatory changes in A_2B or A₃ adenosine receptors (A_2BR or A₃R) expression. The ability of a stable adenosine analogue to activate receptors was shifted to the right by caffeine and in A₁R-A_2AR dHz tissue. Caffeine (0.3 g/L in drinking water for 7-10 days) and A₁R-A_2AR dHz genotype increased locomotor activity (LA) and decreased heart rate, without significantly influencing body temperature. The acute stimulatory effect of a single injection of caffeine was reduced in A₁R-A_2AR dHz mice and in mice treated long term with oral caffeine. Thus, at least some aspects of long-term caffeine use can be mimicked by genetic manipulation the A₁R and A_2AR