We studied nasal mucus flow rate in 12 nonsmoking pairs of monozygotic twins. A radioactivetagged particle was used to measure mucus flow rates at 4 different positions in the nose. A significant correlation was found between repeated measurements in the same subject, but the correlation between measurements within pairs was not significant. The scatter diagram of the measurements of the average mucus flow with saccharine particles indicated a linear relationship within pairs; thus, a constitutional element in the over-all control of nasal mucociliary flow may exist, but the significance level was 0.05 < P < 0.10. The use of the saccharine technique for screening of nasal mucociliary function during occupational exposure to gases or particles is suggested.