Astroglial cells cultured from 8-day-old mouse brain synthesized and secreted nerve growth factor (NGF). An increase in cell density or the withdrawal of serum from the culture medium caused a drastic decrease in the rate of NGF secretion which could be reversed by reculturing at a low cell density or by refeeding with serum-containing culture medium. The cells cultured for two weeks without serum entered the quiescent phase without loss of the activity of an astroglial marker enzyme, glutamine synthetase. These results suggest that NGF secretion by astroglial cells in vitro is regulated in a growth phase-dependent manner. Evidence is also presented to show that NGF secretion is not phase-specific in the cell cycle.