In order to approach the question of how the brain is moulded by environmental stimuli, the neocortex of guinea-pigs just before birth was compared to that of adult animals. In part I of this work we have already seen that the majority of synapses and dendritic spines is already developed before birth in this animal. The investigation was made in precallosal and postcallosal cortex. The samples for electronmicroscopy were taken from the second layer. After birth the following changes have been observed in these areas of the guinea-pig cortex: 1. A slight increase in the thickness of dendrites and dendritic spines (fig. 4). (A changes in the distribution of spines along the dendrite could not be observed; fig. 5). 2. An increase in the number of myelinated fibers, especially in the first layer (fig. 16). 3. An increase in the diameter of the individual synaptic contact area by an average of 10% (fig. 6) which is, however, only significant in the precallosal area. The frequency distribution of synaptic size was similar in prenatal and adult guinea-pigs (fig. 7). 4. A significant increase in the relative number of synapses with postsynaptic perforations (fig. 1c) from 6% to 10% of all synapses. An increase in the thickness of the postsynaptic thickening of asymmetric synapses (fig. 15).