To elucidate the importance of diastolic blood pressure in the definition of salt-sensitive hypertension, we studied 54 male subjects, 36 of whom had untreated, mild essential hypertension. The subjects received a 120 mmol/d Na (as the chloride salt) diet for six days. Thereafter they received a 10 mmol/d Na diet for eight days followed by a 400 mmol/d Na diet for another 8 days. Blood pressure was measured hourly "around the clock" on the last day of each diet; the averaged systolic, diastolic and mean blood pressure values were compared. In 22 subjects diastolic blood pressure increased, when salt intake was increased from 10 to 400 mmol/d. In 18 of these 22 subjects systolic blood pressure increased as well. In 20 subjects, systolic blood pressure increased with salt loading while diastolic blood pressure decreased. In 13 subjects both systolic and diastolic blood pressure decreased with increased salt intake. We defined those subjects showing an increase in diastolic blood pressure as salt-sensitive. If mean blood pressure were used to define salt-sensitivity, 8 of our subjects would have been labeled as salt-sensitive who actually decreased their diastolic blood pressure with salt loading. We suggest that consideration of systolic and diastolic blood pressure responses gives better insight into identifying volume and resistance-related phenomena in salt-sensitive hypertension, than does the consideration of mean blood pressure alone. The definition of salt-sensitivity may require reassessment.