The degree of variability in ventricular refractoriness and factors potentially affecting this variability were evaluated in 80 patients undergoing an electrophysiological study. Each of seven variables (stimulation current, coupling interval of the basic drive train to spontaneous rhythm, pause between determinations, bipolar pacing configuration, bipolar vs unipolar pacing, atrioventricular synchrony, and autonomic tone) was evaluated in a group of ten patients to determine its effects on the reproducibility of refractoriness. Measurements were repeated ten times in every patient under each of two conditions. Five variables had significant effects on the reproducibility of measurements. Pacing at 10 mA was associated with less variability in the determination of ventricular refractoriness than pacing at twice threshold (within-subject variance component 4.5 vs 10.1 msec; P less than 0.001). The mean difference between the longest and shortest determinations of refractory periods (range) was 6.2 msec at 10 mA and 8.6 msec at twice threshold. The use of a conditioning period of pacing and continuous trains (eight beats with a 3-sec pause) rather than a variable pause between serial trials reduced the mean within-subject variance component from 16.5 to 3.3 (P less than 0.001) and the mean range of refractory period determinations from 10.8 to 4.8. The use of the distal rather than the proximal pole as the cathode decreased the mean within-subject variance component from 9.4 to 3.3 (P less than 0.001) and the range of determinations from 6.4 to 5.8 msec. Unipolar pacing was associated with less variability than bipolar pacing (mean within-subject variance component 4.6 vs 6.4; P less than 0.05, mean range 5.0 vs 7.6 msec).(ABSTRACT TRUNCATED AT 250 WORDS)