tion:Applying newly devised model, heritability (VA/VP) of plasma uric acid level, corrected for age and sex and standardized, was estimated at 0.8 in families consisting of twin parents, spouses and children. Correlation between spouses due to common genotype (RHO) was approximately 0.1, and variance due to common familial environment (VEC/VP) was -0.3. ANALYSIS of families of selected twin children and their parents resulted in two estimates of heritability: approximately 0.7 and 0.3, rho being 0.34 and 0.04, and VEC/VP being 0.04 and 0.34, respectively. Regression of IQ (y) on corrected and standardized plasma uric acid level (x) in the twin children was y = 5.56x + 123, correlation being 0.334 (p less than 0.025). The result indicates a genetic BASIS of blood uric acid level, which may have resulted from polymorphisms in purine metabolism PATHWAY, end product of which is uric acid in man. The significant correlation between plasma uric acid level and IQ SUGGESTS a contribution of partly common gene loci to the two quantitative traits.