Objective To investigate the effect ofnarrow-band ultraviolet-B(NB-UVB)(311 nm)on dendrite formation in B16 melanoma cells.Methods B16 melanoma cells were irradiated with various doses of NB-UVB(0,25,50,100,200,300 mJ/cm2).Atier additional culture of varying durations,irradiated cells were harvested and subjected to the observation of morphological changes and cell cytoskeleton F-actin microfilaments by phase contrast microscopy and laser scanning confocal microscopy(LSCM).respectively,and to the detection of cell proliferation bv MTT colorimetric assay.Pull down assay was performed to detect the activity of GTP-RhoAA and GTP-Rac1 in B16 cells before and after UVB irradiation.Results Twenty-four hours after irradiation with UVB of 100 mJ/cm2.an increase was observed in the cell body of B16 cells which appeared in sphericity,as well as in the number of dendrites(P＜0.01)which showed a branch-like appearance.compared with non-irradiated cells which had 2-3 dendrites and obscure branches.LSCM revealed that F-actin microfilaments in B16 cells were well organized with clear textures before irradiation;after irradiation wim NB-UVB of 100 mJ/cm2.stress fibers were disassembled and disrupted and the texture became unclear,which was observed as early as 30 minutes and became more and more evident,and at 6 hours the stress fibers displayed a clumping appearance with obscure textures.Following the irradiation with NB-UVB of 100 mJ/cm2,the expression level of GTP-Rac 1 protein increased at l 5 minutes,and.at 30minutes,reached 2 times of that observed in nonirradiated cells,then decreased a liale,but still remained elevated at 60 minutes and 120 minutes,compared to unirradiated cells;meanwhile.the level of GTP-RhoA dropped a little at 30 minutes,then gradually increased and,at 120 minutes.reached 1.6 times of that observed in unirradiated cells.Conclusion Narrow-band UVB(311 nm)can promote dendrite formation.likely via regulating the expression of GTP-Racl and GTP-RhoA in B16 melanoma cells.