The cerebellar glomerulus was studied by electron microscopic morphometry in female Wistar rats. Age-dependent alterations have been revealed from 3 to 28 mth of age, and the effect of centrophenoxine (CPH) was analyzed in two different patterns of administration. First, 27-mth-old rats were treated daily for 6 wk (acute treatment), and second, 18-mth-old rats were treated 3 times per week for 5 months (chronic treatment). The dose was 100 mg CPH/kg body weight, injected intraperitoneally. The surface density (SV), the numerical density (NV) and the average length (L) of the synaptic junctions were calculated from data obtained on ethanol-phosphotungstic acid stained ultrathin sections. An age-dependent reduction of SV and NV of the synaptic contact zones was found, and the L increased in the oldest animals. CPH-treatment resulted in a marked increase of SV in both types of application, whereas the other two parameters behaved differently in the two groups. The chronic treatment resulted in a significant slowing down of the decrease of NV, whereas L remained invariate. On the contrary, the acute treatment increased L but did not alter significantly NV. The results and the differences between the treatment types are discussed in terms of synaptic plasticity and are interpreted as different manifestations of the same reactive synaptogenetic process.