Rabbits kept under ultraviolet B (UVB)-irradiation respond with increasing serum vitamin D (25(OH)D) concentrations, but it is unknown whether irradiation of the animals or their feed contributes more. Twenty-four New Zealand White rabbits were divided into three groups for a four-week period: the control group (C) received no UVB-exposure and non-irradiated hay (ergocalciferol (vitamin D 2 ) concentration 2.22 µg/100 g dry matter). The direct exposure group (D) was provided with 12 h of UVB-irradiation daily and fed the same hay as group C in shaded areas to prevent UVB-irradiation thereof. The indirect exposure group (I) did not receive direct UVB-irradiation but was fed hay of the same batch that was exposed to 12 h of UVB-irradiation (vitamin D 2 6.06 µg/100 g dry matter). Serum 25(OH)D 2 , 25(OH)D 3 , ionised calcium, total calcium, phosphorus, and magnesium concentrations were measured weekly. There was no systematic effect on serum mineral concentrations. The serum 25(OH)D 2 concentrations were significantly higher in group I compared to groups C and D from the second week onwards. 25(OH)D 3 concentrations increased only in group D, with significant differences to both other groups from the third week onwards, yet at lower magnitudes than the noted increase of 25(OH)D 2 in group I. Total 25(OH)D concentrations were highest in group I, intermediate in group D and lowest in group C. Serum total 25(OH)D concentration was more affected by UVB-irradiation of rabbits' feed than by direct irradiation of the animals themselves. If rabbit serum total 25(OH)D concentrations should be managed, diet manipulation rather than animal UVB-exposure appears to be more effective.