In the present study monopolar and bipolar electrode couplings were compared in electrical stimulation of intradental nerves. In experiments on cats the threshold current values of A- and C-fibres and the jaw-opening reflex were measured with both techniques using different durations of current pulses (0.2-50 msec). In human studies the perception and pain thresholds were determined in the same manner. In experiments on anaesthetized cats 170 intradental nerve units were recorded. Of these 118 were A-fibres (conduction velocity (CV) greater than 2 m/sec) and 52 C-fibres (CV less than or equal to 2 m/sec). The threshold current values (pulse duration 10 msec) of A-fibres were almost 2-fold (12.2 microA, S.D. = 7.9) and of C-fibres over 2-fold (91.3 microA, S.D. = 52.3) with bipolar stimulation compared to monopolar (6.9 microA, S.D. = 7.3 and 42.5 microA, S.D. = 21.5 respectively). The mean threshold of the jaw-opening reflex obtained with bipolar electrode coupling was about twice the value obtained with monopolar (8.2 microA, S.D. = 3.7 and 3.4 microA, S.D. = 0.7 respectively). In human experiments the mean perception threshold was 11.7 microA (S.D. = 6.7) with bipolar and 7.2 microA (S.D. = 3.2) with monopolar stimulation. The mean pain threshold was also higher with bipolar than with monopolar stimulation, 16.0 microA (S.D. = 8.8) and 10.3 microA (S.D. = 4.5) respectively. The subjective sensations were different with these two electrode couplings; the pain (at pain threshold level) induced by bipolar stimulation was experienced as sharp or cutting and with monopolar stimulation as dull or more indefinite. It is concluded that there is a marked difference in the electrical threshold values of pulpal nerves and perception threshold of individual human teeth between monopolar and bipolar tooth stimulation. The thresholds of the A-fibres measured from the cat canine tooth closely correspond to the perception thresholds of the human tooth, but the threshold values of the pulpal C-fibres were significantly higher.