Abstract1,25‐dihydroxyvitamin D3(1,25‐(OH)2D3), a metabolically active form of vitamin D, is shown to increase in a dose‐dependent manner the cellular pool of NGF mRNA in murine L‐929 fibroblasts cultured in a serum‐free medium. This effect can be detected as early as 3 hours after 1,25‐(OH)2D3addition and persists for at least 28 hours. It is accompanied by an enhancement of the amount of NGF‐protein secreted in the culture medium. Since the proto‐oncogenec‐fosappears involved in the regulation of the NGF gene (Mocchetti et al.:Proceedings of the National Academy of Sciences of the United States of America86:3871‐895, 1989; Hengerer et at:Proceedings of the National Academy of Sciences of the United States of America87:3899‐3903, 1990), the effect of 1,25‐(OH)2D3onc‐fosexpression was analysed and compared to that elicited by other inducers of the NGF gene, serum (Wion et al: FEBS‐ Letters 189:37‐41, 1985) and phorbol 12‐myristate 13‐acetate (PMA) (Wion et al:FEBS Letters262:42‐44, 1990). Addition of serum or PMA to L‐929 cells was; rapidly followed by a transient activation of thec‐fosgene. In contrast,c‐fostranscripts remained undetected in the presence of 1,25‐(OH)3D3The failure to find any evidence of c‐fos expression suggests that 1,25‐(OH)2D3could enhance the pool of NGF mRNA by a mechanism independent of thec‐fospathway. The effective concentration range, as low as 10−10M raises the possibility that 1,25‐(OH)2D3could represent one of the serum factors that might contribute to the regulation of the NGF gene in vitro and possibly in vivo. This suggests a possible therapeutic i