The metabolism of steroid hormones in normal and neoplastic tissue has been studied in the human parotid gland. In vitro incubation of homogenate preparations with either testosterone, oestradiol or DHT resulted in the formation of the corresponding 17-keto metabolite as the major product. Little or no metabolism could be seen when androstenedione, oestrone or 5 alpha-androstan-3,17-dione were used as substrates, demonstrating that the 17 beta-hydroxysteroid dehydrogenase (17 beta-HSDH) in this tissue favours an oxidative reaction pathway, 17 beta-HSDH activity in normal and tumour tissue was compared using both testosterone and oestradiol as substrates. The activity varied between individuals but was, in all cases, significantly elevated in tumour tissue when this was compared with normal. In conclusion, (1) the human parotid gland contains steroid metabolizing enzymes which favour an oxidative reaction pathway, and (2) the 17 beta-HSDH activity is significantly elevated in neoplastic tissue.