The synthetic glucocorticoid, triamcinolone acetonide, was found to increase melanogenesis in the human melanoma cell line NEL. Treatment of NEL cells for 24 hr with triamcinolone acetonide (1 X 10(-7) M) increased the activity of the enzyme tyrosinase by 43% and the incorporation of the melanin precursor, L-3,4-dihydroxyphenylalanine, by 23%. Additional studies revealed no change in cyclic AMP levels over an 18-hr test period. A 2-hr preincubation of NEL cells with actinomycin D (10 micrograms/ml) prevented the increase in tyrosinase activity by triamcinolone acetonide. When triamcinolone acetonide was added to a synchronized population of NEL cells, an increase in tyrosinase activity was observed at 16 hr, coinciding with the late S phase of the cell cycle. These results suggest that glucocorticoids are involved in the regulation of melanogenesis in NEL cells by increasing the activity of the rate-controlling enzyme tyrosinase.