Expression in Xenopus oocytes of cloned nicotinic acetylcholine receptors (alpha, beta, gamma and delta subunits of BC3H1 receptor), produced more than one sort of functional receptor. This heterogeneity was detectable neither as heterogeneity in single channel conductance, nor as heterogeneity in the burst length. It was seen most obviously as differences from patch to patch in the maximum fraction of time for which the channels are held open at high acetylcholine concentrations, and it was also detectable as differences in the shut time distributions at low acetylcholine concentrations.