The heart of Python regius is functionally divided so that systemic blood pressure is much higher than pulmonary pressure (6.6+/-1.0 and 0.7+/-0.1 kPa, respectively). The present study shows that force production of cardiac strips from the cavum arteriosum and cavum pulmonale exhibits similar force production when stimulated in vitro. The high systemic blood pressure is caused, therefore, by a thicker ventricular wall surrounding the cavum arteriosum rather than differences in the intrinsic properties of the cardiac tissues. Similarly, there were no differences between the contractile properties of right and left atria. Force production was similar in atria and ventricle but the atria contracted and relaxed much faster than the ventricle. Graded hypoxia markedly reduced twitch force of all four cardiac tissues, and this was most pronounced when PO(2) was below 40 kPa. In contrast, the four cardiac tissues were insensitive to acidosis during normoxia although acidosis increased the sensitivity to hypoxia. Adrenergic stimulation increased twitch force of all cardiac tissues, while cholinergic stimulation only affected the atria and reduced twitch force markedly. In spite of the different oxygen availability of the two sides of the heart, the biochemical and functional properties are alike and the differences may instead be overcome by the coronary blood supply.