Spontaneously hypertensive rats (SHR) were given either 'low' (LNa; 0.5 mmol Na 100 g-1 food), 'control' (CNa; 12 mmol) or 'very high' (vHNa; 120 mmol) sodium diets from 5 to 13-14 weeks of age, to explore how these 240-fold variations in Na intake affected body weight, cardiac, renal and adrenal weights, overall water-electrolyte equilibrium and haemodynamic balance during rest, mental stress and blood loss. Body growth was retarded both in vHNa and LNa SHR presumably reflecting disturbed appetite due to the greatly altered dietary Na contents. Compared with CNa SHR, both cardiac and renal weights 100 g-1 body wt were slightly increased in vHNa and decreased in LNa SHR, with opposite changes of adrenal weights. Total body water, haematocrit and plasma Na-K levels were largely equal in the three groups. Furthermore, cardiac output (CO), stroke volume (SV) and central blood volume (CBV) did not differ significantly between groups; if anything, CO and SV were higher and CBV lower in vHNa and LNa SHR than in CNa SHR. However, while mean arterial pressure (MAP) was only marginally elevated in vHNa compared with CNa SHR, both MAP and total peripheral resistance (TPR) were lowered about 15% in LNa SHR with signs of increased sympathetic activity to the heart also during rest. Despite an apparently normal volume and cardiac output balance in LNa SHR, the latter changes suggest a disturbed neuro-hormonal cardiovascular control.(ABSTRACT TRUNCATED AT 250 WORDS)