Specific ouabain binding, active rubidium transport and sodium-lithium countertransport were studied in erythrocytes of 55 normal weight (BMI less than 27), 10 overweight (27 less than or equal to BMI less than 32) and 27 obese (BMI greater than or equal to 32) Finnish subjects after re-examination of the methods. Intra-assay variation coefficients for these three determinations were 6 percent, 6 percent and 12 percent, respectively. When samples from the same subjects were analyzed again after one month interval no significant differences were obtained between the measurements. However, storing of the cells at +4 degrees C increased Na-K-ATPase mediated rubidium transport about 1.5-fold within one day which may partly account for the discrepancies between the previously reported results. Specific ouabain binding of the overweight group appeared to be slightly lower (13.4 percent, P less than 0.05) whereas that of the obese subjects was slightly higher (8.7 percent, P less than 0.05) in comparison with normal weight subjects. Also in active rubidium transport and sodium-lithium countertransport the values of the obese subjects were significantly higher (P less than 0.01, and P less than 0.05, respectively) than those of the normal weight subjects. Furthermore, there existed a significant correlation between active rubidium transport and body mass index (r = 0.34; P less than 0.001) and also between active rubidium transport and specific ouabain binding (r = 0.67; P less than 0.001). In spite of these differences between obese and normal weight subjects there existed considerable overlapping between the groups, and these changes cannot be used as diagnostic tools in screening persons metabolically susceptible to obesity.