Nine male marathon runners were investigated during habitual training (week 0), after 3 weeks of training break (week 3), and after 2 weeks (week 5) and 4 weeks (week 7) of retraining. Maximal oxygen uptake, body fat (BF), and plasma levels of 25(OH)D3, 1,25(OH)2D3, parathyroid hormone (PTH), calcitonin (CT), albumin, and albumincorrected calcium were determined throughout weeks 0–7. The maximal oxygen uptake decreased after training break and increased during retraining (P=0.002). BF did not change significantly. Plasma 1,25(OH)2D3 was elevated after training break and decreased after 2 and 4 weeks of retraining [week 0: 44.0±3.7 (SEM) pg×1-1; week 3: 52.4±6.0 pg×1-1; week 5: 42.0±2.8 pg×1-1; week 7: 36.9±2.3 pg×1-1; P=0.03]. Plasma 25(OH)D3 did not change significantly. Plasma PTH increased throughout the training break and retraining (week 0: 1.36±0.25 pmol×1-1; week 3: 2.02±0.43 pmol×1-1; week 5: 2.23±0.60 pmol×1-1; week 7: 2.63±0.34 pmol×1-1; P=0.03). Albumincorrected calcium values were transiently decreased during retraining (week 3: 2.77±0.08 mM; week 5: 2.47±0.05 mM; week 7: 2.66±0.07 mM; P=0.01). Plasma CT did not change during training break, but was transiently decreased during retraining (week 0: 9.97±0.39 pmol×1-1; week 3: 9.91±0.37 pmol×1-1; week 5: 8.19±0.50 pmol×1-1; week 7: 9.02±0.45 pmol×1-1; P=0.01). Plasma CT was correlated to albumin (r=0.46, P=0.005), albumin-corrected calcium (r=0.34, P=0.04), and maximal oxygen uptake (r=0.45, P=0.006). Plasma 1,25(OH)2D3 was correlated to 25(OH)D3 (r=0.04, P=0.02), and BF (r=0.50, P=0.002). The described endurance training induced significant changes of plasma 1,25(OH)2D3 and PTH despite only transient changes of albumin-corrected calcium and CT.