The ultrastructure of cardiomyocytes of rabbits was found to be drastically altered at a time coinciding with strong magnetic storms by comparison with that usually observed during quiet geomagnetic conditions. The circadian characteristics of systolic and mean pressure in the left and right ventricles of Chinchilla rabbits were assessed and compared between quiet and stormy magnetic conditions. Experiments repeated during four consecutive seasons at the times of equinoxes and solstices in the absence of magnetic storms were also examined for any circannual and/or transannual variation. The results have been interpreted in the broader context of non-photic influences on the circulation, fully supporting the presence of non-photic effects. The recording of magnetic activity in the laboratory, and until this is possible, the consultation of the physicists' routine recording of geomagnetic indices should become a sine qua non, since, as shown herein, magnetic storms can override the effect of the usually dominant synchronizer, the alternation of light and darkness. © 2004 Elsevier SAS. All rights reserved.