A 3 years study was conducted to determine the evolution of blood lead concentrations (PbB) and free erythrocyte protoporphyrins (FEP) as indicators of absorption and biochemical damage in children (7-12 years old) living within one kilometer of a metallurgic factory that processes lead in Torreón, Coahuila, México. In addition, neuromuscular conduction velocity, motor coordination and IQ where determined as indicators of physiological damage. During this period of time the children showed increased lead blood concentrations of 19.2 +/- 4.5 micrograms dl-1 at the beginning of the study to 27.5 +/- 4.9 micrograms dl-1 at the end. Likewise, FEP increased from 56.6 +/- 20.0 micrograms dl-1 to 92.9 +/- 28.9 micrograms dl-1. Physiological damage was evident since motor coefficients as well as IQ tests were reduced significantly when compared to children (7-12 years old) living 4.5 km away from the factory who showed PbB = 8.9 +/- 1.3 micrograms dl-1 and FEP = 16.9 +/- 4.7 micrograms dl-1 throughout the study. Based on these data it was possible to define three different groups with regards to lead toxicity sensitivity.