Human skeletal muscle adapts to microgravity-induced disuse by atrophy and weakness, switch of fibre types (from slow to fast twitch fibres), changes in muscle energy metabolism and metabolic demand (1) and capillarisation (2). We hypothesize that a change in capillarisation precedes muscle fibre atrophy during disuse. Bed rest s a well-accepted model of either inactivity induced by deconditioning or simulated microgravity in healthy humans. There is some evidence that milk-based proteins, such as whey proteins, can act as an effective countermeasure to alleviate or attenuate the atrophy during prolonged bed rest. Here we explored the changes in capillarisation in response to 21 days of medium-term bed rest with and without countermeasure (Whey protein diet). Muscle biopsies were taken from the soleus (SOL) and vastus lateralis (VL) muscles before (pre) and after bed rest (post) with or without whey protein supplementation (n = 5, each). Fibre types and capillaries were identified by immunohistochemical co-staining of muscle section with anti-myosin type1 and lectin. Analysis was done with the method of capillary domains (3). Statistical analysis was done by repeated measures ANOVA. Ethical approval was obtained from the local Ethics Committee and each participant provided informed consent before enrolment in the study. Surprisingly, bed rest did neither affect fibre type composition nor fibre size in both SOL and VL muscles. Capillary density was, however, 10% less in the SOL (p<0.01), but not in the VL muscle after bed rest. Despite the reduced capillary density in the SOL, the heterogeneity of capillary spacing was not significantly changed in both muscles. Whey protein did not have a major effect on the impact of bed-rest-induced muscle adaptations. In conclusion, our results show that medium-term bed rest induced a reduction in capillary density in the soleus but not in the vastus lateralis muscle. The absence of significant muscle fibre atrophy suggests that a reduction in capillary supply may precede fibre atrophy in disuse.