Comparison of K+ channels in mollusk and mammalian neurons has been made to elucidate their fundamental properties. Using patch clamp cell-attached configuration, K+ channels in isolated snail neurons were separated into three subtypes: with big (BKC), medium (MKC) and small (SKC) unitary conductances. BKC and MKC were activated at -30 mV and SKC at more negative potentials. BKC and MKC proved sensitive to TEA, whereas SKC were sensitive to 4-AP. Cd2+ in the pipet decreased unitary conductance of BKC by 55% and of MKC by about 31%. Bath application of 5-HT selectively suppressed MKC. It is suggested that BKC can be referred to large conductance Ca(2+)-dependent K+ currents (KCa), MKC to intermediate conductance KCa and SKC channels comply with the characteristics of A current of mammals. These data show that KCa and A currents may be the most general types of currents generated by K+ channels.