Physiological properties of acetylcholine receptors on muscle cells at very early stages of ontogeny were compared with those of cells at later stages. Two changes were observed that contributed to an overall shortening of the mean open time of single-channels. First, there was a shift in the relative proportions of two receptor types with different conductances and mean open times, such that the contribution of receptors with large conductance and short open time increased as development proceeded. Second, there was a sharp reduction in the mean open time of channels having small conductance, with no similar change in channels having large conductance.