Background: Vitamin D is a steroid-hormone involved in a wide range of physiologicalprocesses in the human body. It has more recently been proposed that muscle cells are able tostore vitamin D and secrete the hormone into the blood stream during muscle contractions. Fewstudies have previously investigated the acute responses on vitamin D levels after exercise,however evidence suggests an exercise-induced elevation of vitamin D parameters. It isadditionally proposed an association between vitamin D levels and various fitness parameters.Aim and Hypothesis: We sought to investigate the acute response on circulating levels ofvitamin D, immediately (0 hours), 3 hours and 24 hours after one session of high-intensityinterval training (HIIT) or strength training, and to compare the response between these twomodalities. Further, it was explored if an association between vitamin D response and fitnessparameters was present, along with possible gender differences. It was hypothesized that serumlevels of vitamin D would increase after both modalities, and that the exercise-induced responsewas correlated with fitness level.Methods: 36 young, healthy participants completed one session of both HIIT and strengthtraining. Blood samples were collected at baseline, 0 hours, 3 hours, and 24 hours after bothsessions and later analysed for 25-hydroxyvitamin D3 (25(OH)D), 1.25-dihydroxyvitamin D3(1.25(OH)2D), and vitamin D binding protein (DBP) at all time points. All participants weretested for maximal oxygen consumption (VO2max), maximal strength (1RM) and rate of forcedevelopment (RFD).Results: Strength training induced a significant elevation in 25(OH)D both 0 hours (p<0.001)and 3 hours (p<0.001) after exercise compared to baseline. HIIT elevated 25(OH)D levelsimmediately after the exercise session (p<0.001). There was a rise in circulating levels of1.25(OH)2D from 0 hours to 3 hours after endurance exercise (p<0.001), that even exceededbaseline levels (p<0.006). The levels sustained elevated in blood samples drawn 24 hoursafter(p<0.013). DBP was additionally elevated 0 hours after endurance exercise(p<0.047).Conclusion: One session of either HIIT or strength training was determined to have a directimpact on circulating levels of vitamin D metabolites. Both sessions displayed a significantelevation in 25(OH)D levels immediately after the exercise, with strength training beingseemingly more effective. Present findings might enlighten exercise as an additionalintervention strategy when discussing vitamin D influencing factors.