Dexamethasone is known to decrease the pool of nerve growth factor (NGF) mRNA in various experimental systems. The negative regulatory effect of the glucocorticoid was first observed in mouse fibroblast-like L929 cells, and was subsequently reported to take place in many experimental systems, including in vivo following sciatic nerve injury. Conversely, another steroid hormone, 1,25-dihydroxy-vitamin D3 (1,25-(OH)2D3) was recently reported to promote NGF synthesis in mouse L929 cells. The present work was undertaken to investigate the effect of the concomitant addition of both steroids to L929 cells. Measurements of NGF mRNA and assays of the mature protein secreted by the cells provide evidence that the negative regulation exerted by dexamethasone may be counteracted in a dose-dependent manner by the positive action of 1,25-(OH)2D3, and vice versa. Therefore, the expression of the NGF gene can be regulated in a subtle way by the balance between the two steroids. It may be expected on the basis of these observations that in tissues that are responsive to both hormones, administration of 1,25-(OH)2D3 should be able to reverse the down-regulation of NGF synthesis elicited by glucocorticoids.