1. The development of layer V pyramidal neurons is analysed quantitatively in albino rat temporal ("auditory") cortex from the 1st to the 90th postnatal days (12 stages). The length of apical dendrites, the number of primary dendrites and the total amount of apical dendrite spines are registered in Golgi-Cox preparations (55 animals). The diameters of the nucleus, length and width of the perikaryon and the relation between nucleus and perikaryon are measured in Nissl-series (45 animals). 2. Two types of development can be recognised by the examined parameters: --Length of apical dendrites, number of primary dendrites and of apical dendrite spines aspire more or less continuously to a maximum value. --Sizes of nucleus and perikaryon show intermediately a higher value than the terminal one ("overshooting growth"). 3. The postnatal development of the parameters suggests that the dendritic growth (also after initiated phase) starts from the perikaryon and relates with dendritic neuroplasmic flow. 4. In order to give general statements about the evolution of layer V pyramidal neuron's rates of growth are counted and their degree of maturity is determined. The biggest rates of growth are reached up to the 12th day post partum. At this time the pyramidal neurons have a relatively high degree of maturity. 5. There are two periods with especially marked alterations of structure of the layer V pyramidal neurons. These alterations are regarded as morphokineses according to Scharf. I. The morphological changes between the 8th and the 12th day are regarded as "morphokinesis as a reaction to planned crises" (2.2., according to Scharf 1970). In this case the critical situation is the beginning of hearing of the young rats, which is to be prepared. II. The morphological changes between the 24th and 36th day take place in the critical period of primary socialization (Scott et al. 1974). This could be understood as "morphokinesis as a reaction to environmental influences" (2.1., according to Scharf 1970). In this period it is possible, that the layer V pyramidal neurons of the temporal cortex of the rat play a role in learning and memory.