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1. Action potentials were recorded from aggregates of heart cells prepared from 3- to 7-day chick embryos. At 3 days the maximum rate of rise (+ Vmax) was insensitive to TTX; at 7 days it was considerably reduced by TTX. 2. In the presence of TTX the action potential overshoot was dependent on [Ca]0; the results may be fitted using constant field theory and assuming that the membrane is over a hundred times more permeable to Ca than to Na or K. 3. An increase in stimulation rate in the range 0.2-2 Hz led to an increase in both overshoot and + Vmax. This effect was not seen after addition of 20 mM-tetraethylammonium ions, nor when Sr was substituted for Ca in the external medium. We suggest that these rate-dependent changes may result from partial inactivation of an outward K current.