The dependence of contraction in the atrial myocardium on the stimulation pattern was studied in normoxic and hypoxic conditions. Isotonic contractions were measured from left atrial muscle strips of the rabbit. At rapid stimulation hypoxia produced a negative inotropic effect which disappeared at slower stimulation and became a positive inotropic one. The potentiation of a first beat after a pause in stimulation was enhanced by hypoxia because the contractions were stronger depressed at steady state than after a pause. At all pacing patterns the relative maximum rate of shortening and re-lengthening were independent of the oxygen-supply. After application of epinephrine in hypoxia they were enhanced and so was peak shortening. The findings suggest an impaired or altered function of the sarcolemma only under the degrees of hypoxia chosen.