Blood chemistry values were measured from 31 harbor porpoises (Phocoena phocoena) released from herring weirs in the Bay of Fundy, Canada. Sodium, phosphorus, calcium, chloride, magnesium, total protein, albumin, globulin, urea, cholesterol, serum osmolality, and alanine aminotransferase levels fell within ranges reported for captive harbor porpoises and other odontocetes. Glucose, potassium, creatine kinase, aspartate aminotransferase, hemoglobin, thyroxine, bilirubin, and alkaline phosphatase levels were generally higher than those reported for captive odontocetes. Cortisol and creatinine levels were significantly higher (P= 0.0356 and 0.0174, respectively) in porpoises that were handled for longer periods and fitted with electronic tags (mean cortisol value = 314 ± 107 nmol/L, mean creatinine value = 94 ± 14 μmol/L) than those receiving roto-tags (mean cortisol value = 222 ± 82 nmol/L, mean creatinine value = 75 ± 15 μmol/L). A more controlled study of duration of handling, time spent out of water, degree of manipulation, and method of tag attachment is required to identify which factors are responsible for the stress response.