Our aim was to estimate causal relationships of genetic factors and different specific environmental factors in determining the level of cardiac autonomic modulation i.e. heart rate variability (HRV) in healthy male twins and male twins with chronic diseases. The subjects were 208 monozygotic (MZ, 104 healthy) and 296 dizygotic (DZ, 173 healthy) male twins. A structured interview was used to obtain data on lifetime exposures of occupational loading, regularly performed leisure-time sport activities, coffee consumption, smoking history and chronic diseases from age 12 through the present. A 5-minute electrocardiogram at supine rest was recorded for the HRV analyses. In univariate statistical analyses based on genetic models with additive genetic, dominance genetic and unique environment effects, genetic effects accounted for 31-57% of HRV variance. In multivariate statistical analysis, body mass index, fat percentage, coffee consumption, smoking, medication and chronic diseases were associated with different HRV variables accounting for 1-11% of their variance. Occupational physical loading and leisure-time sport activities did not account for variation in any HRV variable. However, in the subgroup analysis of healthy and diseased twins, occupational loading explained 4% of the variability in heart periods. Otherwise, the interaction between the health status and genetic effects was significant for only two HRV variables. In conclusion, genetic factors accounted for a major proportion of the interindividual differences in HRV with no remarkable effect of health status. No single behavioral determinant appeared to have a major influence on HRV. The effects of medication and diseases may mask the minimal effect of occupational loading on HRV.