Tricaine methanesulfonate (MS-222) is an anesthetic commonly used to reduce fish stress during transport or sampling. The exposure of blacknose daceRhinichthys atratulusto MS-222 at concentrations of 300 mg/L or 500 mg/L for 9 min or less did not raise hematocrit levels above those of controls. Hematocrit levels tended to be somewhat lower in experimental groups than in controls; however the only significant difference (P= 0.044) among treatments occurred between the control fish (35% ± 2.3;N= 6) and fish exposed to 500 mg/L for 3 min (30% ± 1.8;N= 7). This difference suggests that initial exposure to MS-222 may cause stress or, less likely, that the anesthetic has some inherent hemodilution effect. Workers who monitor environmental acidification may be concerned with the hematocrit of the acid-sensitive blacknose dace because hematocrit increases during acid stress. The use of MS-222 to ease hematocrit sampling should not elevate measurements.