1. Following the finding of high levels of oestrogen receptor proteins in pancreatic carcinoma tissue, two enzymes involved in sex-steroid biosynthetic pathways, aromatase and 5 alpha-reductase, have been measured. 2. Activities of aromatase, which converts testosterone into oestradiol, comparable with those found in pre-menopausal uterus (P less than 0.5) were found in all seven samples of pancreatic carcinoma tissue, and in a pooled sample of foetal pancreas. Measurable but significantly lower activities (P less than 0.001) of aromatase were found in seven specimens of normal pancreas. 3. 5 alpha-Reductase activity, which converts testosterone into the more potent androgen 5 alpha-dihydrotestosterone, was found in malignant pancreatic tissue at approximately half the level found in prostatic tissue (P less than 0.01) and at almost twice the levels found in either normal adult or pooled foetal pancreatic tissue (P less than 0.01). 4. These findings suggest that sex steroids are involved in foetal and adult pancreatic physiology. 5. Since these enzyme pathways are present in pancreatic carcinoma at greater levels than those in normal adult pancreas, it is possible that agents known to interfere with steroid metabolism could be of value in the treatment of this tumour.