A field survey of 71 salesmen (22-60 years) in a machinery manufacturing company was conducted to investigate the effect of working hours on biological functions related to the cardiovascular system. The subjects were divided into four groups by age, and those in each age group were further divided into shorter (SWH) and longer (LWH) working hour subgroups by weekly working hours. Rates of complaints of subjective fatigue for LWH were significantly higher than those for SWH on the whole. Although the mean amplitude of respiratory sinus arrhythmia at rest decreased with age, no significant difference between SWH and LWH was found in this function. Systolic blood pressure for LWH was significantly higher than that for SWH in the 50-60 year group. The serum total cholesterol level for LWH was significantly lower than that for SWH in the 40-49 year group. Comparison of biological functions related to cardiovascular system was also made between fatigue complaint and no-complaint subgroups. Significant differences were found between the two subgroups: systolic blood pressure was higher and the total cholesterol level was lower in the fatigue complaint subgroups. Summarizing our results, it appeared that long working hours might increase systolic blood pressure and lower the total cholesterol level due to fatigue.