When rat peritoneal mast cells were exposed to ultraviolet (UV) light (UVA, UVB and UVC), histamine release was evoked in a dose (intensityxtime) dependent manner. The potency order of UV light in inducing the histamine release was UVC>UVB>>UVA. In this study, we focused on the effect of ultraviolet B (UVB) on histamine release from rat mast cells. The UVB-induced histamine release occurred at doses higher than 7.8 kJ m-2, even at 4°C. At a UVB dose of 18.8 kJ m-2, where a 51.9±4.8% histamine release and a 58.8±6.8% degranulation took place, Trypan blue-stained cells accounted for 14.4±1.3% of the cells, and the lactate dehydrogenase (LDH) release was about 4.9±2.8%. This suggests that the membrane permeability to low molecular weight substances was increased by UVB exposure. The UVB-induced histamine release was inhibited by ascorbic acid at concentrations higher than 500 μM, suggesting the involvement of a radical reaction in the process. The UVB-induced histamine release was enhanced by some phenothiazine compounds, i.e., promethazine, trimeprazine, mequitazine, chlorpromazine, trifluoperazine, ethopropazine and thioridazine. We conclude that the phototoxicity of phenothiazine compounds may be due in part to an enhancement of UVB-induced histamine release from mast cells. Copyright (C) 1999 Elsevier Science B.V.