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Objectives: To assess the patterns of early postnatal physiological adaptation and maturation in intrauterine growth retarded (IUGR) infants by measuring changes in sleeping deep body temperature, heart rate, and concentrations of urinary cortisol. Setting: At home. Patients: Sixty five IUGR babies and 127 controls matched for sex, social class, and levels of parental smoking. Results: Night time sleeping deep body temperature, heart rate, and cortisol excretion fell with age, eventually establishing an adult type diurnal rhythm of physiological function. Minimum overnight temperature showed a linear decline with age (p, 0.001), but the IUGR infants and the controls had significant differences in intercept (p = 0.007) and slope (p = 0.02). The estimated rate of decline per week was 0.020 C ̊ for IUGR infants and 0.031 C ̊ for controls. Maximum temperature did not show similar changes. IUGR infants had a mean (SE) age adjusted minimum overnight heart rate that was 4.2 (1.5) beats/min (p = 0.005) higher than controls. Overnight cortisol/creatinine ratios declined with age at a rate of 4.1 % per week (log ratio 20.421 (0.0165), p = 0.01), but the ratio for IUGR infants was on average 42 % higher (log ratio 0.35 (0.11), p = 0.002) than for controls of the same age. Morning cortiso