Ectopic spiral wave sources were produced in pieces of 15 X 15 mm of rabbit atrium by an induction of a single properly timed premature impulse. The cycle length of the ectopic source (T) gradually increased immediately after its appearance. It was shown that T growth is determined by the rise of refractoriness (R). It was found that besides the well known phenomenon of R shortening after f increase there is an opposite process of R lengthening at high driving rates (f > 4--5 sec--1). A hypothesis is advanced that the growth of refractoriness found is one of the death mechanisms of spiral wave ectopic sources in the heart.
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