slow inward current in aggregates of neonatal rat heart cells and its contribution to the steady state currentvoltage relationship in voltage clamped neonatal rat heart cells a transient current is observed during depolarizing potential steps which was identified as slow inward current isi by its range of activation by its reversal potential of approximately 50 mv and by its sensitivity to d600 or low external ca2 this isi activates too fast to be detected by the present methods which implies that activation is completed within milliseconds the time constant of inactivation was weakly potential dependent and less than 30 ms between 40 mv and 20 mv the f infinity curve of isi had a sigmoidal shape with 90 and 10 values near 50 mv and 10 mv respectively half maximum was at 25 mv from double pulse experiments an estimate was obtained of the potential dependence and amplitude of steady state isi a maximum was expected around 30 mv steady state isi appears to be present indeed in steady state current voltage relations as the relative minimum at 30 mv in such relations is abolished by 5 x 107 gml d600 currents tails during hyperpolarizing steps from prepulse potentials near 0 mv are potential dependent in a way expected when isi contributes to these current tails by a decrease in inactivation moreover the current tails are diminished by d600 or co2 consequences of steady state isi are discussed