Contrasting data are published on the effects of high salt intake (between 300 and 660 mmol/d) on Na balance and fluid retention. In some studies high levels of NaCl intake (400, 440, 550 and 660 mmol/d) led to positive Na balances without fluid retention. To test the relevance of different baseline NaCl intake levels on changes in metabolic water, Na, K, chloride and acid–base balance, a 28 d clinical trial (‘Salty Life 6’) was carried out in a metabolic ward. Nine healthy male volunteers (aged 25·7 (sd 3·1) years; body mass (BM) 71·4 (sd 4·0) kg) participated in the present study. Four consecutive levels of NaCl intake: low (6 d, 0·7 mmol NaCl/kg BM per d), average normal (6 d, 2·8 mmol NaCl/kg BM per d), high (10 d, 7·7 mmol NaCl/kg BM per d), and low again (6 d, 0·7 mmol NaCl/kg BM per d) were tested. Urine osmolality, extracellular volume (ECV) and plasma volume (PV), cumulative metabolic Na, K, chloride and fluid balances, mRNA expression of two glycosaminoglycan (GAG) polymerisation genes, capillary blood pH, bicarbonate and base excess were measured. During average normal NaCl intake, 193 (sem 19) mmol Na were retained and ECV (+2·02 (sem 0·31) litres; P < 0·001) and PV (+0·57 (sem 0·13) litres; P < 0·001) increased. During high NaCl intake, 244 (sem 77) mmol Na were retained but ECV did not increase (ECV − 0·54 (sem 0·30) litres, P = 0·089; PV +0·27 (sem 0·25) litres, P = 0·283). mRNA expression of GAG polymerisation genes increased with rise in NaCl intake, while pH (P < 0·01) and bicarbonate (P < 0·001) levels decreased. We conclude that a high NaCl intake may increase GAG synthesis; this might play a role in osmotically inactive Na retention in humans.