The influence of environmental factors on synaptogenesis in the rat hippocampus (stratum radiatum of CA 1) was studied by the aid of quantitative electron microscopy. Three groups of newborn Wistar-rats were exposed to different environmental conditions (sensory deprivation and stimulation, respectively). The first group was raised in complete darkness while the second was exposed to permanent noise. The third group (controls) was reared under normal laboratory conditions. The animals were sacrificed either on the 15th or on the 35th postnatal day and prepared for electron microscopy. In the 15 day old dark raised animals the number of synapses in the stratum radiatum of CA 1 was significantly decreased by 24% in comparison with the normal. On the other hand, 15 day old rats exposed to permanent noise exhibit significantly more synapses (+ 28%) in the stratum radiatum of CA 1 (the mean number of synapses/25 micrometer 2 is 4,30 +/- 1,85 in the controls, 3,29 +/- 1,66 in the dark raised group and 5,52 +/- 2,35 in the animals subjected to permanent noise). No significant differences in the number of synapses between experimental animals and controls could be found in 35 day old rats. In accordance with earlier studies on the development of dendritic spines in hippocampal neurons the results of the present paper indicate an environment-dependent differentiation of synaptic structures in the hippocampus of the rat during the early postnatal period.