The effects of serum deprivation, of dibutyryl-cyclic AMP (dBcAMP) and of theophylline on the morphology of cultured new-born rat astroblasts have been studied using Eagle's basal medium (BME) or Eagle's minimum essential medium (MEM) as culture media. Serum deprivation had no effect on cells cultured in BME, while in MEM, deprivation induced a rapid morphological transformation involving the appearance of multiple processes. This phenomenon was rapidly reversible when serum was again added. In serum-supplemented BME, dB-cAMP (1 mM) and theophylline (1 mM) had no effect. In serum-supplement MEM, theophylline (1 mM) had no effect while dB-cAMP (1 mM) induced a slower and poorly reversible morphological alteration. On the other hand cells in serum-free BME showed multiple processes after addition of dB-cAMP (1 mM) or theophylline (1 mM). This rapid alteration was completely reversed either by removal of dB-cAMP and theophylline or by addition of serum.