Deuterium labelling experiments have shown that the thermal conversion of tetramethyl 4aH-benzo[c]quinolizine-1,2,3,4-tetracarboxylate into the 1H-isomer is an intramolecular process, whereas the photochemical conversion in methanol involves deuterium exchange with the solvent. Mechanisms are discussed.Dimethyl acetylenedicarboxylate, when treated with 2-isopropyl-, 2-styryl-, and 2,3-trimethylene-quinoline, and with 1,2,3,4-tetrahydroacridine gave 4a-alkyl-4aH-benzo[c]quinolizines and other products. Irradiation of these quinolizines, and other quinolizines with bridgehead hydrogen atoms or alkyl groups and obtained from pyridines and isoquinoline, caused migration of the bridgehead group to position 1 in sterically favourable cases. Pyrroloazepines were also formed in some instances.