Arrhythmias developing in isolated rabbit atria following the cooling of the perfusion solution were studied by multielectrode mapping. In 10 out of the 12 initially invulnerable preparations, the cooling to 27 +/- +/- 0.7'C induced arrhythmias. The effect was reversible, disappearing with the temperature normalization. Mapping showed that in 60% of the cases the arrhythmias were caused by the emergence of the leading cycles (functionally determined re-entry). In 40% of the cases, the re-entry was not demonstrable by mapping. However, arrhythmias in these experiments developed at the same temperature and had the same period as the leading cycles. The appearance of arrhythmias was closely correlated with a decrease in the wave length which strongly suggests the re-entry mechanism of hypothermic arrhythmias.