1. In embryonic ascidian muscle, outward K+ currents develop in two stages: the initial expression of a slowly activating, voltage-gated K+ current (IKv) near the time of neurulation is followed about 6 h later by a rapidly activating calcium-activated K+ current (IK(Ca)). During this 6 h interval, inward Ca2+ currents (ICa) appear and the inward rectifier (IK(IR)), the sole resting conductance, is transiently downregulated. These events predict a period of spontaneous activity. The following experiments were designed to test this prediction and to examine the relevance of spontaneous activity for muscle cell development. 2. By recording activity in cell-attached patches, we have found that muscle cells generate spontaneous action potentials during this 6 h window of time when IK(IR) is downregulated and outward K+ currents are slow. Action potentials occur at a mean frequency of 13.9 min-1.3. When activity is blocked by the transient application of the Ca2+ channel blocker Cd2+, IK(Ca) fails to develop. This disruption is specific for IK(Ca): IK(IR) and ICa develop normally in activity-blocked cells. Application of Cd2+ either before or after the window of activity has no effect. 4. The reappearance of IK(IR) and the development of IK(Ca) and the mature form of ICa are all prevented by transcription blockers, with a sensitive period corresponding to the period of activity. 5. These data show that, although the expression of three channel types depends on transcription during the period of spontaneous activity, only the development of IK(Ca) depends on activity.