SummaryThis study was designed to measure acetaminophen (paracetamol) levels in tears, and to compare it to serum levels. Paracetamol levels were measured in 20 paired tears and serum samples from 10 healthy volunteers, 1 and 2 hours after ingesting 1.5 g paracetamol. Tears were collected using glass microcapillary tubes while stimulating the conjunctiva with a small sponge placed in the lower fornix. Blood samples were taken simultaneously. The samples were analyzed for paracetamol levels using homogeneous enzyme immunoassay. Tears and serum paracetamol levels 1 hour after ingestion were 16.3 &mgr;g/mL ± 7.2 (mean ± SD), and 21.4 &mgr;g/mL ± 7.7 (mean ± SD) respectively. Tears and serum levels 2 hours after ingestion were 14.4 &mgr;g/mL ± 7.8 (mean ± SD), and 17 &mgr;g/mL ± 7.6 (mean ± SD) respectively. Tears and serum paracetamol levels of all the 20 paired samples (1 h and 2 h after ingestion) were 15.35 &mgr;g/mL ± 7.4, and 19.25 &mgr;g/mL ± 7.8, respectively (mean ± SD). There was a strong and highly significant correlation between paracetamol levels in serum and in tears 1 and 2 hours after ingestion (r= 0.8, p = 0.005,r= 0.85, p = 0.002 respectively). Mean ± SD ratio of tears/serum paracetamol levels 1 hour and 2 hours after ingestion were 0.77 ± 0.21 and 0.81 ± 0.25 respectively. &Dgr; tears (difference in mean levels at 1 and 2 hours) paracetamol levels is significantly correlated with &Dgr; serum levels (r= 0.7, p = 0.025). A reliable, convenient, and feasible noninvasive method is described for measuring paracetamol in tears. There is no information in the literature about tears paracetamol secretion, and little information of tears drugs concentration.