Quantitative morphological changes of the developing Purkinje cells were studied from 6 to 90 postnatal (PN) days in the IVth lobule of vermis in the cerebellum of rats. The soma size (mean diameter) of Purkinje cells increased rapidly between 6 PN (on average 10 microns) and 18 PN (about 17 microns) days; it did not change between 18 and 25 PN days, but increased moderately again between 25 and 48 PN days (22-23 microns) and stabilized on the same value. In contrast, the number of Purkinje cells/100 microns (the "linear density") decreased rapidly from 6 to 18 PN days. The molecular layer area belonging to 1 Purkinje cell increased rapidly from 6 to 25 PN days (from about 370 to 6,200 microns 2) and less rapidly between PN days 30 to 48 (up to 9,300 microns 2), followed by a moderate decrease at PN day 90 (about 6,600 microns 2). The volume belonging to 1 Purkinje cell dendritic arbor was about 5,500 microns 3 at PN day 6, 93,000 microns 3 at PN day 25, and 100,000 microns 3 at PN day 90. The numerical density of dendritic spines in the molecular layer showed a biphasic curve: a rapid increase from PN days 6 to 21 followed by a significant but short decrease at PN day 25, moderate rise from PN days 25 to 48, and a subsequent decline between PN days 48 and 90. The number of spines belonging to 1 Purkinje cell showed two developmental "peaks": the first peak at 21 PN days was moderate (5.6 x 10(4) spines/Purkinje cell) while the second maximum at 48 PN days was more significant (1.2 x 10(5) spines/Purkinje cell), which then declined to 6.3 x 10(4) spines/Purkinje cell at PN day 90. It is suggested that the temporary overproduction and the following decline in the number of Purkinje dendritic spines during the development of the cerebellar cortex may be the morphological indicator of the dynamics of synaptogenetic and of synaptic stabilization processes.