Identified neurons of the stomatogastric ganglion of the crab Cancer borealis were voltage-clamped, and the current densities of three K+ currents were measured. The current densities of each of the three K+ currents varied twofold to fivefold in inferior cardiac (IC) neurons from different animals. Conventionally, this degree of variability has been attributed to experimental artifacts. Instead, we suggest that it reflects a natural variability that may be related to an underlying process of plasticity. First, we found that there is no fixed ratio among the three K+ currents. Second, we found that several hours of stimulation with depolarizing current pulses (0.5 sec duration at 1 Hz) altered the current density of the Ca2+-dependent outward current, IK(Ca), and the transient outward current, IA. This stimulation paradigm mimics the normal pattern of activity for these neurons. The effect of stimulation on the IA current density was eliminated when Ca2+ influx was blocked by extracellular Cd2+. In contrast, the K+ current densities of the lateral pyloric (LP) neuron were unaffected by the same pattern of stimulation, and the currents expressed by both the IC and the LP neurons were insensitive to hyperpolarizing pulses at the same frequency. We conclude that the conductance densities expressed by neurons may vary continually depending on the recent history of electrical activity in the preparation, and that intracellular Ca2+ may play a role in the processes by which activity influences the regulation of current densities in neurons.