When day old chicks are trained to avoid pecking at a bright bead coated with methyl anthranilate, many neurochemical changes, both transient and longer lasting, have been found. These include an increased fucose incorporation in vivo into particulate glycoproteins, which persists for at least 24 hrs after training. We have now developed an in vitro method for studying fucose incorporation and have been able to replicate effects of training found in vivo. Chick forebrain slices incubated at 42 degrees in a glucose containing-medium incorporate L-[U14C]fucose linearly for up to 3 hrs at rates of 30-35 nmol/g prot/hr. Incorporation was only 60% inhibited by cycloheximide indicating that some fucosylation is occurring on pre-existing proteins. Fucose incorporation was compared in slices from trained and control chicks and, as in vivo, a 16% increase in incorporation into the right forebrain base of trained birds was found. This increase was confined to the microsomal fraction. When cycloheximide was added to the incubation medium, the enhanced fucose incorporation in slices from trained birds was still observed.