Action of serotonin (5-HT) on single Ca2+ channel activity was studied in identified neurons of snail Helix pomatia. Only one type of Ca2+ channels of 5 pS unitary conductance was determined under patch-clamp cell-attached mode. Kinetic analysis have shown a monotonically, declining distribution of channel open times (OT) with mean time constant of 0.2 ms. The distribution of channel closed times (CT) could be fitted by double-exponential curve with time constants 1 and 12 ms. We established that 5-HT acts on Ca2+ channel activity indirectly via cytoplasm. 5-HT prolonged the OT (up to 0.3 ms) and shortened the CT proportionally for both constants to 0.4 and 6 ms correspondingly. A conclusion is made that enhancement of Ca2+ macro-current by 5-HT is determined by kinetic changes. increase of the number of active channels. and increase of the probability of OT. At the same time the transmitter did not affect the unitary channel conductance. (C) 2002 Elsevier Science (USA). All rights reserved.