In vitro studies with skin samples or pure precursors of cholecalciferol indicated that cholecalciferol synthesis during UV light exposure is a non-linear process. However, in vitro studies indicate nothing about the relationship between sunlight exposure and physiological cholecalciferol status of living organisms. Due to the lack of cholecalciferol in plant material, this relationship is important for herbivores including domestic cattle, particularly in organic agriculture, because the use of synthetic additives, like cholecalciferol, is restricted in order to fulfil the principles of sustainable organic production. The major physiological metabolite of cholecalciferol is the liver-derived 25-hydroxycholecalciferol (25(OH)D3). The purpose of the present study was to determine the relationship between sunlight exposure and 25(OH)D3 status in vivo in large herbivores during mid-summer at 56°N. Five groups of cows were given access to pasture during 15, 30, 75, 150 or 300 min daily for 28 d in June and plasma analysed for 25(OH)D3. Animals allowed 15, 30 or 75 min of daily access to pasture showed a declining linear relationship between plasma 25(OH)D3 and sampling day in contrast to animals allowed 150 or 300 min of pasture access which showed linear increasing plasma 25(OH)D3 status. Determined from the slopes of 25(OH)D3 concentration curves within treatments, breakeven for maintaining the initial 25(OH)D3 status of 45 nmol/l was 90 min pasture access per d during summer at 56°N.