Thymidine kinase and DNA polymerase enzyme activities were measured in epididymal adipose tissue from rats of 12 to 182 days of age. Both enzymes showed highest specific activity during the suckling period; by 35 days of age both thymidine kinase and DNA polymerase enzyme activities had decreased to stable lower levels. The activities of the two proliferative enzymes resembled the pattern of [3H]thymidine incorporation into preadipocytes shown by Greenwood and Hirsch (1) and the data support the concept that a pool of preadipocytes develops during the first 4 to 5 weeks postnatally. Further, the thymidine kinase and DNA polymerase enzyme activities were correlated with the rate of DNA accretion in the preadipocyte fraction of the tissue. Since thymidine kinase activity can be measured in 20 to 40 mg of tissue. Since thymidine kinase activity can be measured in 20 to 40 mg of tissue, the technique can be adapted for measurement of enzyme levels in human or animal biopsy samples when radio-isotope studies are not advisable or only small quantities of tissue are available.