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The changes of the coronary flows and of the cardiac nucleotide metabolism during acidosis and during alkalosis were studied in 50 perfused guinea pig hearts with and without hypoxia. At pH 7.0 the coronary flows increased, and at pH 7.8 a significant reduction of the flows took place. At 20% O2, acidosis elicited a further flow increase, whereas alkalosis inhibited the flow increase produced by hyoxia. The increases after adenosine injections and after coronary occlusions were greater during acidosis and smaller during alkalosis than at pH 7.4. The cardiac nucleotide contents did not clearly differ from the controls whereas adenosine exhibits higher levels in acidotic hearts. Alkalosis always induced a decreased production of adenine nucleoside irrespective of the presence or the absence of hypoxia. At 20% O2 a decreased ATP level and increased ADP- and CrP-contents could be observed during alkalosis.