This paper describes a technique which allows the determination of the shape of action potentials initiated spontaneously, orthodromically or antidromically, without the distorting effects on shape that may be produced by changes in latency of activation or by signal averaging techniques. Software-controlled stimulation allows precise control of parameters for antidromic collision testing. The method allows selective sampling of a single action potential from a rapidly discharging cell, from a population of similar cells or from a evoked action potential among spontaneous activity. It minimizes jitter in the antidromic peri-event histogram by presenting an antidromic stimulus at precise and reproducible delays following either a spontaneous or orthodromically evoked action potential.