Pattern of irregular type of dorsal root discharge (DRD) in non-anaesthetized spinal cats was inferred from the distribution of its interspike intervals. Interspike interval histograms were compiled from antidromic spike potentials of irregular DRD which were recorded in the central ends of 33 single dorsal root fibres of L7 dorsal root. The majority of histograms was unimodal. The mean preferred interval of irregular DRD which indicated the most frequently occurring interspike interval was 6.1 +/- 0.5 ms. The shortest and longest intervals were 3.7 +/- 0.3 ms and 45.9 +/- 4.4 ms, respectively. The coefficient of symmetry of the histogram was 0.06, indicating highly unsymmetrical distribution of intervals of irregular DRD. Conduction velocity of dorsal root fibres carrying irregular antidromic discharge amounted to 52.3 +/- 5.3 m/s (n = 23). There were no significant correlations between three analysed interspike intervals and conduction velocity. The results are discussed in view of the hypothesis of the modulating effect of antidromic discharge on the afferent inflow. Comparison of preferred interspike interval of irregular DRD with the existing data on the rate of orthodromic activity in afferent nerve fibres suggests that irregular antidromic discharge on many occasions is able to block othodromic impulses by collision.