Newly hatched chicks of the domestic fowl (White Leghorn) were imprinted to an acoustic stimulus (Group I: 400 Hz, 3 bursts per s, Group II: 900 Hz, 2 bursts per s) and tested in a straight runway with loudspeakers behind two opposite goal boxes. Those chicks were considered imprinted which headed for the imprinting stimulus in an approach test and subsequently preferred it to a new stimulus (imprinting stimulus of the other group) in a simultaneous discrimination test. On day 7 after hatching (after the sensitive phase) imprinted chicks and naive controls were injected with 2-[14C]deoxyglucose (2DG) and exposed to the imprinting stimulus. Autoradiographic analysis of their brains revealed 3 well demarcated areas of increased 2DG accumulation in the rostral forebrain of imprinted chicks compared to controls: HAD in the rostral Wulst; MNH, an auditory area in the rostromedial neostriatum and hyperstriatum ventrale; LNH in the rostrolateral neostriatum and hyperstriatum ventrale. Analysis of these brain areas in 7-day-old acoustically imprinted and control animals with the Golgi-Cox method revealed a highly significant reduction of spine frequency of a large neostriatal neuron type in MNH of imprinted chicks. An additional Golgi-Cox analysis was carried out with chicks imprinted on a broody hen, i.e. on the whole spectrum of natural stimuli. In that group spine frequency of the same neuron type was between that of acoustically imprinted and control animals. A hypothesis of filial imprinting is presented which considers spine loss as a crucial mechanism.