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SUMMARY1. Tail preparations, containing myotomal muscle and associated spinal cord, were isolated from embryos and tadpoles of Xenopus laevis between stages 25 and 49 (1-15-12 days) and were pulse-labelled with 125I-a-bungarotoxin (125IIaBT) so that the half-life (Ti) of their acetylcholine receptors (AChRs) could be estimated in organ culture. 4. At stages 47-49 (5'5-12 days) the decline in radioactivity for the entire population of AChRs was fitted well by a double exponential. Assuming a T1 of 50 h for the extrajunctional AChRs and 210 h for the junctional AChRs, the correlation coefficient (r) was 0-9947 + 0i0014 (mean + S.E.M.; n = 14) and junctional AChRs were estimated to comprise 80 + 3 % of the entire population. Similar analysis, as well as experiments in which the degradation of junctional AChRs was assessed by pulselabelling with fluorescent cc-bungarotoxin, suggested that at earlier stages of development the junctional AChRs have a shorter T and comprise a smaller fraction of the entire population.5. The developmental increase in Ti occurred even when animals were raised in the anaesthetic tricaine or in tetrodotoxin, conditions which abolished all motor activity.6. Developmental increases in T 'also occurred in culture but were smaller than those in vivo. The increases in culture did not occur amongst those AChRs which were pre-labelled with 125IIaBT.7. It is concluded that in Xenopus myotomal muscle the T1 of junctional AChRs begins to increase within a day after the onset of innervation and that the increase does not require nerve or muscle impulse activity. We suggest, among other possibilities, that it may depend upon incorporation of a different molecular species of AChR into the postsynaptic membrane.

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