The development of electrical activity in aggregates of dissociated hydra cells was investigated. Both the number of events and the distribution of amplitude of electrical pulses during the process of regeneration were determined. Correlations between electrical pulses at two separate positions on an aggregate were also examined as regeneration progressed. The main results were as follows: From the measured frequency and amplitude of pulses, together with the correlations between pulses at two different sites, we could observe the way in which contraction pulses developed in regenerating aggregates. The percentage of the electrical pulses of large amplitude increased about 20 hr after the start of regeneration. The correlation between electrical pulses measured at two sites separated by 0.3 mm also increased at about 20 hr, reaching about 80% at 40 hr. The results suggest the possible details of the reorganization of the conduction system for contraction pulses among cells in a regenerating aggregate of hydra cells.