33 acetylcholine perfusions were carried out on 14 neurones RPal of a snail, 25 serotonin perfusions on 15 neurones, and 10 dopamine perfusions on 12 neurones. Frequency of unit activity (FUA) was analysed as well as its irregularity (IUA). The latter was calculated by a new method of successive comparison of the lengths of interspike intervals which accounts more adequately for causes of lengthening of each interval in comparison to evaluation of dispersion and coefficient of variation. During the neurotransmitters' action, IUA did not depend on the background IUA and the duration of its return to the background level from the moment of the beginning of neurotransmitter action. At low neurotransmitter concentrations, FUA changed less than by 0.5 spike/s. IUA assumed any values from 8 to 68 per cent and lasted for different periods from 22 to 448 s depending on the kind of neurotransmitter. At high neurotransmitter concentrations, FUA changed more than by 1 spike/s. IUA took only high values from 33 to 80 per cent and lasted only from 18 to 62 s. The value of IUA reflects the tension of neuronal processes directed to preservation of stable rhythmical activity.