The expression of calcium-activated potassium currents (IK(Ca)), delayed outward rectifier potassium currents (IK(slow)), and transient outward currents (IA) was studied during the development of the nervous system of the leech using the whole-cell patch-clamp recording technique. Dissociated cells were isolated from leech embryos between stage E7 and E16 and maintained in primary culture. K+ currents were recorded at E7, when only few anterior ganglia had formed beneath the primordial mouth. IK(slow) was present in all cells tested, while IK(Ca) was expressed in only 67% of the cells studied. Even as early as E7, different types of IK(Ca) have been found. Neither frequency of occurrence nor the charge density of IK(Ca) showed significant changes between E7 and E16. The density of IK(slow), however, increased by a factor of two between E7 and E8, which resulted in a significant increase in the total K+ current of these cells. This rise in potassium outward current developed in parallel with the appearance of Na+ and Ca2+ inward currents (Schirrmacher and Deitmer: J Exp Biol 155:435-453, 1991) during early development, shaping the electrical excitability in embryonic leech neurones. I(A) could be separated by its voltage-dependence and pharmacological properties. The current was detected at stage E9, when all 32 ganglia are formed in the embryo. The frequency of occurrence of I(A) increased from 16% at E9 to 70% at E15. The channel density, steady state inactivation, and kinetics showed no significant changes during development.