Phenylcarbene and adamantylidene have been generated photochemically from the corresponding diazirines within lipid bilayers. Reasonable yields of labeled fatty acid side chains have been observed. The products have been characterized by gas chromatography-mass spectrometry and derive both from the insertion of the carbene into carbon-hydrogen bonds of saturated fatty acids and from the addition of the carbene to the carbon-carbon double bonds of unsaturated fatty acids. In contrast to the results found using phenylnitrene, the lipid labeling by carbene is not reduced by the water-soluble scavenger glutath ione. Carbenes generated from diazirines are evidently superior reagents for the photolabeling of lipids and should be useful for identifying the intrinsic hydrophobic sections of membrane proteins.