. Behavioral and sensory health effects due to low levels of occupational exposure to elemental mercury (Hg) vapor have not been well characterized below 50 ug Hg/L in urine. Adverse effects have been reported at mean levels as low as 26 ug Hg/L but in the absence of reliable dose-effect relationships, a safe level in urine and air remains to be determined. The hypothesis is that recent (sub-chronic) and cumulative (chronic) doses of low level elemental Hg may adversely affect: a) motor function (simple reaction time, finger-tapping, the Hand Steadiness Battery, and acceleration tremor), b) information processing (choice reaction time), c) memory/attention (digit span, Sternberg, and visual memory recurrent figures), d) cognitive skill (Trailmaking B, Raven matrices, switching attention, coding), e) mood (POMS and the Symptom Check List 90 Revised (SCL-90-R), f) symptoms, and g) sensory function (visual contrast sensitivity, vibrotactile sensitivity, and postural sway). The control tests are vocabulary and visual acuity. The objectives are to determine for the first time whether potential deficits are associated with low doses and to accurately identify a threshold level of effect. Potential nervous system deficits are expected to appear at recent and cumulative mercury exposure levels significantly lower than those required to produce renal toxicity A repeated measures cross-sectional study design will be conducted among 200 dentists practicing in Washington State and 200 dental assistants, selected from a population of >3500 practices using first morning void urine as a screening measure. The distribution of urinary levels will be used to select a representative sample (twenty five percentiles) of eligible dentists where the highest and the lowest strata will be over sampled to increase the range of exposures and to control for age across exposures. The assistants will be selected from participating dental offices. Test sessions will be counter-balanced for seasonal exposure by testing half the group in winter then summer and the other half in reverse order, six months apart. Within subject variability will be used to distinguish sub-chronic effects due to recent exposures from chronic effects. A 24-hour urinary mercury level is a stable estimate of dose, but will be compared to the amount in blood and hair. An index of cumulative dose will be based on work histories weighted for work practices. Renal assessments in urine include measurement of glutathione transferase isoenzymes, GST-alpha and GST-pi, retinol binding protein, and albumin, reflecting tubular structural integrity, functional integrity, and glomerular functional integrity, respectively. Completion of this project will describe neuro-behavioral effects associated with occupational exposures below 50 micrograms/liter encompassing the proposed biologic thresholds of 25 and 50 micrograms/liter. The results will define whether potential deficits are chronic or sub-chronic in nature by using well characterized indices of exposure and dose. A safe threshold may also be identified on an anticipated continuum from sub-clinical to clinical effects. These results are both applicable to the occupational health of the dental community and to public health policy on the impact of elemental mercury exposure in the general population.