Age-related changes in epoxide hydrolase (EH) activity in the liver, kidney, lung, and intestine were studied in male C57BL/6 mice of 1 through 30 months of age. Hepatic cytosolic EH activity increased until 15 months after which there was a decline of 59% during senescence (30 months). Hepatic EH activity in the mitochondrial fraction increased until 4 months and decreased thereafter with a 43% decline by 30 months. The hepatic microsomal EH activity increased until 6 months followed by decline of 32% by 30 months. All of these increases and declines were statistically significant. Renal cytosolic EH showed maximum activity at 6 months after which the activity decreased significantly with age. However, renal EH activity in the mitochondrial fraction, in general, did not change substantially with age. Although changes in renal microsomal EH activity were small, the decrease in activity at 9-18 months was significantly lower than at 1 month and 23-30 months. EH activity in the cytosolic, mitochondrial and microsomal fractions of kidney was 2.5-, 2-, and 10-fold less, respectively, than that found in similar subcellular fractions of liver. Cytosolic EH activity in lung and intestine and lung microsomal EH showed variations with age. The intestinal microsomal EH was not detectable under the experimental conditions used.