1. The hypoxia-induced changes of contractile force, frequency and diastolic tension in isolated atria from guinea pig, rat and rabbit were studied together with tissue content of adenine nucleotides, creatine phosphate and lactic acid. 2. Increasing hypoxia induced a progressive reduction of contractility in guinea pig, rat and rabbit atria. Hypoxia also induced a progressive reduction of frequency in rat atria, whereas atrial rate decreased significantly only during drastic hypoxia in guinea pig and rabbit. In spontaneously beating atria, hypoxia increased diastolic tension only in rat. 3. After 30 min of drastic hypoxia, ATP decreased in guinea pig and rabbit but not in rat atria. Creatine phosphate decreased and lactic acid increased in all three species. 4. These data suggest that: (a) hypoxia-induced changes in atrial function and in metabolic content are species-dependent; (b) hypoxia-induced inhibition of atrial activity could represent protection aimed at saving energy.