Dependent on the level of occupational exposure to tetraethyl lead, the occurrence of early signs of toxicity and the urinary excretion of triethyl lead, diethyl lead and total lead compounds were investigated This was done in the following cohorts in the province of Hubei, China: 277 workers at gasoline depots exposed to gasoline, 36 traffic policemen exposed to automobile exhaust and 342 public office workers (virtually non-exposed controls) Mean external tetraethyl lead exposure concentrations were 84 8 g/m 3 (as Pb) for the gasoline depot workers, 5 2 g/m 3 for traffic police and 1 1 g/m 3 for the controls No significant subclinical indications of organic lead toxicity were found in the group of traffic policemen compared with the controls In the cohort of gasoline workers, however, there was a statistical increase (vs controls) in the frequency of appearance of tremor and of sinus bradycardia When the cohort of gasoline workers was divided into subgroups of different ranges of exposure, dose-dependence was noted In general, the urinary excretion of triethyl lead was very low compared to that of diethyl lead, which appears to be a sensitive and specific indicator of exposure to tetraethyl lead; total lead excretion did not correlate well with actual external tetraethyl lead exposure On the basis of these data it seems that current occupational exposure limits for tetraethyl lead are inadequate and need to be revised In addition, a biological limit, based on urinary diethyl lead excretion, may be proposed.