Gangliosides are sialic acid-containing glycosphingolipids that play a variety of important functions in neurons. The main purpose of this study was to determine the a/b ratio of gangliosides in different rat brain regions (cerebral cortex, hypothalamus, caudate nucleus, hippocampus, thalamus and cerebellum) after prolonged diazepam treatment. Male Wistar rats were maintained on a nutritionally adequate diet and diazepam was administered in a dose of 10 mg/kg day for 180 days. Total gangliosides were extracted according to Harth and the total ganglioside-NeuAc content was determined by Svennerholm's resorcinol method, modified by Miettinen and Takki-Luukkainen. The a/b ratio remained unchanged in rat hypothalamus, thalamus and cerebellum. It was slightly decreased in the caudate nucleus and hippocampus, but this was not statistically significant. A drastic decrease (p<0.01) in ganglioside content, compared to control animals, was found in rat cerebral cortex. Ganglioside a/b profile did not change significantly in most of the brain regions (except in cerebral cortex), which suggests that adaptive changes occurred upon prolonged exposure to diazepam, in order to maintain the physiological ratio of ganglioside a- and b-series in distinct brain areas.