The proteins D2 (N-CAM) and D3, both markers for brain neuronal membranes, MM, a marker for mitochondria, and CM, a cytoplasmic marker protein, were studied in the prefrontal (anteromedial) cortex and "inferotemporal" (Te2) cortex of rats by crossed immunoelectrophoresis. Three experimental groups were investigated: Rats trained to criterion in a visual pattern discrimination test (learning), those run as yoked controls and finally, rats kept in individual cages and not subjected to any training (passive). Statistical analysis indicated that behavioral procedures and marker proteins contributed significantly to the variation. Further analysis indicated that the significant changes occurred in the D3- and MM-protein and that both the learning and yoked control groups had significantly increased concentrations of these two proteins when compared to the passive group. Furthermore, the concentration of D3- and MM-protein in the yoked control group was significantly higher than that of the learning group. The results seem to indicate that changes in concentration of these proteins can be more easily related to the activity of "searching for an adequate behavioral strategy" than to the formation of an "engram".