SummaryPlasma and urine uric acid levels were measured in 37 pairs of twins aged 11 or 12 years and their parents. Effect of age differed between sexes and between plasma and urine. Assuming linear regression on age, the uric acid levels were adjusted for a fixed age, then standardized with mean 0 and standard deviation 1. The standardized values for plasma yielded a high MZ twin correlation (0.8429), a high mid-parent-offspring regression (0.7516) and a high parent-offspring regression (0.5100). Because of a high husband-wife correlation (0.3771) a model was used for estimating parameters, where effects of assortative mating and/or common within-family environment were incorporated. In plasma two sets of estimates were obtained: VA/Vp=0.7064, 0.3251; VD/Vp=0.1010, 0.1769; and VEC (variance due to common within-family environment)/Vp=0.0362, 0.3416. In urine VA/Vt,=0.4125, Vo/Vp=0.3556 and VEc/Vp=--0.1106. It was concluded that heritability of uric acid level was not less than 0.3 in plasma and not less than 0.4 in urine. The plasma uric acid level and IQ was positively correlated (between 0.17 and 0.34), but not significant.