This study examines the effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on proliferation of SH-SY5Y human neuroblastoma cells and demonstrates, for the first time, the presence of vitamin D receptor (VDR) in this cell line. Cell number showed a significant decrease, when the cells were incubated with 1 or 10 nM 1,25(OH)2D3 for 24, 48, 72, 96 and 144 h, while 100 nM 1,25(OH)2D3 was ineffective after 24 and 96 h incubation. The highest inhibition (about 35%) was observed after 72 h treatment with the hormone at the three concentrations used. Protein content per cell increased, in comparison with controls, after treatment of SH-SY5Y neuroblastoma cells with 1,25(OH)2D3, at the three concentrations, up to 96 h incubation. 1, 10 or 100 nM 1,25(OH)2D3 positively affected [3H]thymidine incorporation after treatment of the cells for 48 and 72 h, while, after 24 h, only 10 nM 1,25(OH)2D3 exerted a stimulatory action. To study the expression of the VDR gene, Northern blot analysis was performed. Subconfluent SH-SY5Y neuroblastoma cells were treated for 24 h with medium containing 10 nM 1,25(OH)2D3 or vehicle alone. A main transcript of an approximate size of 4.5 kb, present either in controls or in cells incubated with the hormone, was revealed. A limited increase in VDR mRNA levels was observed in the cells treated with 1,25(OH)2D3, fetal bovine serum or forskolin. Only slight differences in morphology were perceived between SH-SY5Y cultures maintained with or without 10 nM 1,25(OH)2D3 for 7 days.