This study sought to determine the relation between mean RR intervals as well as its circadian rhythm and non-invasive markers of atrioventricular (AV) node refractoriness and concealed AV conduction obtained from Lorenz-plot (LP) analysis of 10-min and 24-h ECGs. In 29 patients with persistent atrial fibrillation (AF) the 1.0-s intercept of the lower envelope (LE1.0) of the plot and the degree of scatter (S) above the envelope were measured as markers for refractoriness and concealed conduction. Mean 24-h RR was independently associated with LE1.0 and S calculated from 10-min ECGs. In 48% patients, a significant circadian RR rhythm was present, which was independently associated with both S and LE1.0 amplitudes. AV conduction properties obtained from LP analysis of 10-min ECGs are useful for predicting mean 24-h heart rate in AF. The degree of variation in refractoriness and concealed conduction are determinants of significant circadian heart rate variation in AF.