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The uncharged photoactivable probe 2-[H-3]diazofluorene ([H-3]DAF) was used to examine structural changes in the Torpedo californica nicotinic acetylcholine receptor (AChR) ion channel induced by agonists. Photoincorporation of [H-3]DAF into the AChR consisted of the following two components: a nonspecific component consistent with incorporation into residues situated at the lipid-protein interface, and a specific component, inhibitable by noncompetitive antagonists and localized to the M2 hydrophobic segments of AChR subunits. The nonspecific [3H]DAF incorporation was characterized in the M4 segment of each AChR subunit. The observed distribution and periodicity of labeled residues reinforce the conclusion that the M4 segments are organized as transmembrane a-helices with a common "face" of each helix in contact with lipid, Within the M2 segments, in the absence of agonist [H-3]DAF specifically labeled homologous residues beta Val-261 and delta Val-269, with incorporation into beta Val-269 at a 5-fold greater efficiency than into beta Val-261. This observation, coupled with the lack of detectable incorporation into alpha-M2 including the homologous alpha Val-255, indicates that within the resting channel [H-3]DAF is bound with its photoreactive diazo group oriented toward delta Val-269. In the presence of agonist, there is an similar to 90% reduction in the labeling of beta Val-261 and delta Val-269 accompanied by specific incorporation into residues (beta Leu-257, beta Ala-258, delta Ser-262, and delta Leu-265) situated 1 or 2 turns of an alpha-helix closer to the cytoplasmic end of the M2 segments. The results provide a further characterization of agonist-induced rearrangements of the M2 (ion channel) domain of the AChR.