OBJECTIVE It has been shown that the distribution of Na+ and Ca2+ in various cells is abnormal in patients with untreated primary hypertension, indicating an altered membrane permeability in these cells. This would activate certain ion pumps and thereby enhance ATP turnover. We investigated possible alterations in energy economy of skeletal muscle tissue. METHODS Skeletal muscle energetics were studied in vitro and in vivo in patients with untreated primary hypertension. Phosphocreatine (PCr), energy charge (EC) and total adenylate values were assessed. METHODS The study was performed at the outpatient clinic of a general hospital and at a university clinical chemistry department and at a specialized bioenergetic laboratory. METHODS Altogether, 17 patients with untreated primary hypertension were examined together with matched, healthy and normotensive controls with normal body-mass index. METHODS Skeletal muscle biopsies were obtained from 10 patients and 10 controls for analysis of high energy phosphate compounds. Another seven patients were enrolled for in vivo NMR spectroscopy. RESULTS We found a decrease of 30% (P < 0.01) of PCr content in the patients, whilst EC and total adenylates were unchanged. Nuclear magnetic resonance spectroscopy showed an abnormal decrease of PCr during exercise followed by a markedly slower regeneration of PCr during post-exercise recovery parallelled by a slower recovery of pH. This phenomenon was mirrored by a more pronounced decrease of ATP/Pi in patients during exercise and a slower recovery of ATP/Pi. CONCLUSIONS The data are compatible with an increased ATP turnover in skeletal muscle cells of patients with untreated primary hypertension although ATP was favoured and kept at a normal resting level at the expense of the PCr store.