An operant conditioning study was carried out on Wistar rat pups of brain-growth-spurt age to assess the plasticity of apical dendritic branching and the numerical spine densities of CA3 pyramidal neurons of the hippocampus. In rapid Golgi-stained material we studied the effects of operant conditioning commencing at 16 days of age and terminating on the 23rd day of age. The data of the learning group (L) were compared with those of the sham experimental control group (E) and of the control group reared under standard conditions (S). The results revealed a significant (ANOVA F test) increase in the number of branching points of the dendrites of the learning group compared with the other groups. The grand averages of spine densities also showed an overall increase in the learning group over the sham-experimental and standard groups. Segmental comparisons revealed a more interesting observation that significant changes in the spine densities of the learning and sham-experimental groups occurred in some segments of dendrites but not in all. A few segments showed moderate spine decreases also, compared with the control group. The spine differences of segments were mainly in the stalked types. Excrescences which are spines on which mossy fibers are known to synapse increased in both learning and sham-experimental groups, but not significantly (ANOVA). The dendritic branching and spines of nonmossy inputs of certain dendritic segments significantly increased after the learning experience in the hippocampus of the growing brain.