Primary cultures of glia contain a high glial fibrillary acidic protein level and exhibit important glutamine synthetase activity. They take up serotonin via a high affinity carrier-mediated system with a Km of micromolar range. The Km of this transport process does not vary during cell growth or maturation; however, during the last period of morphological change induced by dibutyryl cyclic AMP, an increase in Vmax is observed. Chlorimipramine, fluoxetine and fenfluramine at 10-4 M inhibit this uptake. 3H-5-HT still binds to partially purified astrocytic membranes on a single type of site. During growth, neither KD nor Bmax were modified. During the maturation period, KD decreased to about 50% of its control level. Methysergide inhibits that binding.