The basal (BMR) to sleeping metabolic rate (SMR) ratio might represent an estimate of the activation of the nervous system (central/sympathetic) from sleeping to basal state. Since this activation might be influenced by the degree of obesity, and might be different between sexes, we retrospectively analysed energy expenditure data collected for a large number of subjects. Twenty-four hour energy expenditure (24EE), BMR and SMR were measured in a respiratory chamber in 122 Caucasians (63 males/59 females, 32 +/- 10 years, 94 +/- 33 kg, 29 +/- 11% fat) (means +/- s.d.) and in 123 Pima Indians (68 males/55 females, 29 +/- 7 years, 100 +/- 25 kg, 34 +/- 9% fat). The BMR/SMR ratio varied greatly between individuals (1.05 +/- 0.08; range 0.87-1.34). In Pima Indians, BMR/SMR was inversely correlated to both fat mass (r = -0.26; P < 0.01) and BMI (r = -0.22; P < 0.05), whereas, in Caucasians, BMR/SMR was inversely correlated to waist/thigh circumference ratio (r = -0.28; P < 0.01). On average, the BMR/SMR was higher in Pima Indians than in Caucasians (1.06 +/- 0.08 vs. 1.03 +/- 0.07, P < 0.01) and higher in Pima Indian males than in Pima Indian females (1.08 +/- 0.09 vs. 1.04 +/- 0.06, P < 0.05). Studies are needed to investigate whether these differences in the increase in energy expenditure from the sleeping to the basal state are related to differences in the activation of the nervous system and/or to other metabolic factors.