The sublethal toxicity of inorganic (HgCl₂) and organic (CH₃HgCl) mercury chloride was studied in coturnix (Coturnix japonica) by feeding them mercuric compounds (CH₃HgCl at concentrations of 0.125,0.5,2 and 8 ppm; HgCl₂ at 0.5, 2, 8 and 32 ppm) in ad libitum diets from hatching to adulthood. Differences of response to the mercurials were compared on the basis of selected indicator enzymes and plasma chemistries. Comparisons of response to equivalent concentrations of the two mercurials and dose-response relationships were made at 1,3,5,7 and 9 weeks. Changes of activity were detected for brain acetylcholinesterase (AChE) and the plasma enzymes aspartate aminotransferase (ASAT), butyrylcholinesterase (BChE), lactate dehydrogenase (LDH) and ornithine carbamoyl transferase (OCT). Changes of ASAT, LDH and OCT were then quantified by probit analysis and the mercurials were compared through their median effective concentrations (EC₅₀). This quantal procedure was based on the establishment of normal control values for each enzyme and then classifying mercury-treated outliers (more than ± 2 SD) as respondents. The EC₅₀ values at 9 weeks for ASAT, LDH and OCT, respectively, were 9, 3 and 63 ppm for HgCl₂, and 5, 1 and 4 ppm for CH₃HgCl. These results provided the basis for two hazard indices that were calculated by dividing the EC₅₀ into the oral LD₅₀ and the 5-d dietary LC₅₀. Mercury also had contradictory effects on gonadal maturation in both sexes.