We examined the associations between sodium and blood pressure (BP), and between 3-methylhistidine (3MH)(a marker of animal protein intake) and BP in four Chinese population samples (Guiyang, Guangzhou, Shanghai and Shijiazhuang). This work was a constituent part of the World Health Organization (WHO) Cardiovascular Disease and Alimentary Comparison (CARDIAC) Study. Each population sample consisted of 100 men and 100 women aged 48-56yr and randomly selected using a cross-sectional study design. Twenty-four hour urine samples were collected. Urinary sodium and potassium excretion levels were measured by the flame photometry method, and 3MH was measured using a Hitachi Amino Acid Analyzer 835 (Hitachi, Ibaraki, Japan). After excluding subjects who did not complete the 24-h urine collection (as assessed by urinary creatinine excretion in relation to weight), the total study group included 314 men and 355 women. The results showed that (1) Sodium was positively, and 3MH negatively associated with systolic and diastolic BP (SBP, DBP) in both the total sample and in those who were not administered anti-hypertensive drugs; these associations were all significant (p<0.05), and remained so after adjustment for age, sex, body mass index [BMI, weight (kg)/height (m²)], alcohol intake and potassium excretion. Sodium and 3MH were also observed to exert a combined effect on BPs. In general, subjects who had higher sodium and lower 3MH levels had higher mean SBP and DBP. This combined effect was particularly clear on SBP. (3) A positive association between sodium and BP, and a negative association between 3MH and BP were also shown in subjects who had BMI less than 26kg/m². In conclusion, the study confirmed and further extended previous observations on the study of salt and animal protein intake in relation to BP in middle-aged Chinese. The results support recommendations for a reduction in high salt intake for the control of high BP in the general population and in those with lower BMI. The results also provide important evidence that adequate animal protein intake may have a favorable effect on BP. (Hypertens Res 2000; 23:151-157)