When an impulse arrives at an area of impaired conductivity, a slowly rising electrotonic potential may bring the distal tissue to threshold after a delay imposed by the passive electrical properties of the system and by the time-dependent changes of these properties during diastole. This phenomenon can be demonstrated in Purkinje strands in which an area of depressed conductivity has been induced by the impedance of a sucrose gap and can be mimicked by the application of relatively long current pulses of low amplitude. The functional refractory period, defined as the shortest interval between two distal responses both propagated across the gap, was determined by the application of premature stimuli at progressively earlier intervals. The time course of the recovery of excitability as well as the conduction intervals could be varied almost at will by manipulating the electric impedance between proximal and distal ends of the fiber. When the time of activation of the distal end across the gap exceeded the absolute refractory period of the proximal segment, the impulse reflected back as a closely coupled premature beat. Time-dependent changes in the passive electrical properties of the depressed segment may set the conditions for reflection. The results suggest the possibility of reflection as a mechanism for premature beats and demonstrate obligatory shifts in the patterns of premature reentrant activity accompanying changes in basic cycle length. These experiments provide important clues for the distinction between reentrant and parasystolic mechanisms.