Nerve growth factor (NGF) is a protein indispensible for the normal development, survival and maintenance of function of peripheral sympathetic neurons throughout life, and of spinal sensory neurons during a brief period of their development [ 1,2] . Beyond the general promotion of growth and the stimulation of fiber outgrowth from sympathetic neurons [l] , the selective induction of tyrosine hydroxylase (TH) and dopamine fi-hydroxylase (DBH) is one of the most characteristic biochemical effects of NGF on adrenergic neurons and adrenal chromaffm cells [3,4] . Recently, a clonal cell line (PC1 2) was established from a transplantable rat adrenal pheochromocytoma [ 51. This cell line shows a high degree of differentiation, involving synthesis, storage, release and uptake of the adrenergic transmitter [5,6], reflecting functional properties of adrenergic neurons. Moreover, as a further characteristic property of adrenergic neurons, PC 12 cells respond with fiber outgrowth to NGF [S] , although they lack a selective induction of TH, another characteristic response to NGF [7] . So far, neuronal fiber outgrowth is the only common response of all target cells to NGF, whereas a corresponding common biochemical response has not yet been described. Since PC12 cells respond to NGF with fiber outgrowth but not with selective TH induction, they seemed to be suitable for study of the biochemical responses to physiological levels of NGF which are common to all target cells of NGF. We report that NGF produces a specific increase in the activity of ornithine decarboxylase (EC 4.1 .l .17, L-ornithine carboxy-lyase) (ODC), the rate-limiting enzyme in polyamine biosynthesis [8] , This increase of ODC can be abolished by cycloheximide indicating that the rise ln ODC activity is protein synthesisdependent. In addition, we report that NGF-mediated increase in ODC activity is a cyclic AMP independent process.