An increased activity of Na⁺/H⁺ antiport has been reported in leukocytes and fibroblasts from insulin-dependent diabetic (IDDM) patients with nephropathy. To test whether a similar abnormality is present in fibroblasts from non-insulin-dependent diabetic (NIDDM) patients with microalbuminuria and hypertension, we examined intracellular pH_i and Na⁺/H⁺ antiport activity, using the pH sensitive dye 2, 7-bis (2-carboxyethyl-5(6)-carboxyfluorescein (BCECF), in cultured skin fibroblasts obtained from 34 NIDDM patients, divided into four groups based upon whether they had microalbuminuria or hypertension, or both: Group 1, nine NIDDM patients with microalbuminuria and hypertension. Group 2, nine NIDDM patients with hypertension and normal albumin excretion rate. Group 3, seven NIDDM patients with microalbuminuria and normal blood pressure. Group 4, nine NIDDM patients with normal blood pressure and normal albumin excretion rate. Nine normal subjects served as control group. Resting pH_i was more alkaline in fibroblasts from Group 1 (7.22±0.03; p<0.05), Group 2 (7.21±0.02; p<0.05) and Group 3 (7.19±0.02, p=0.17) than in Group 4 and normal subjects. This was due to higher V_max values of Na⁺/H⁺ antiport activity in cultured fibroblasts from Group 1 (52.1±5.3 mmol H⁺/min; p<0.05), Group 2 (57.7±8.3; p<0.05) and Group 3 (60.6±7.4, p<0.05) than those from Group 4 (31.2±3.6) or control subjects (31.3±3.5). The intracellular pH for half-maximal activation, Hill coefficient and buffering power capacity was similar in all the groups. These data suggest that in vitro phenotypic abnormalities of long-term cultured fibroblasts from NIDDM patients with microalbuminuria and/or hypertension are likely to be, at least in part, independent of the degree of metabolic control in vivo and to be an intrinsic feature of these cells.