Slices from the forebrains of day-old chicks represent a highly active in vitro protein-synthesising system. The in vitro incorporation of L-[14C]leucine into protein of slices was estimated to be 2.5 mmol/mg protein/h. Incorporation was linear over 90 min of incubation and was suppressed by 92% by 1 mM cycloheximide. The highest incorporation was into microsomal and cell-soluble fractions. Under the electron microscope, slices appeared vacuolated near the cut surfaces, but well preserved internally (greater than 40 micron from the edge). Autoradiography showed that radioactivity was incorporated evenly across the slice with no decrease in label in the central part of the tissue. The rate of incorporation was only weakly dependent on leucine concentration in the medium (0.04-1 mM). Addition of a mixture of unlabelled amino acids (1 mM) produced a 20-50% inhibition of incorporation of radioactive L-leucine depending on the amino acids involved. In slices prepared from chicks 1 h after training on a one-trial passive avoidance paradigm, L-[14C]leucine incorporation was 23% higher (p less than 0.01) in the forebrain roof than in slices from control chicks. This figure is comparable to the one previously reported in vivo. Subcellular fractionation of incubated slices from the forebrain roof of trained and control birds revealed that the increased protein synthesis was due mainly to an elevated leucine incorporation into the soluble fraction.