Three pituitary adenomas obtained from patients wtih acromegaly were maintained in organ explant culture for a period of 12 days in a defined culture medium. Lyophilized culture medium from each was then examined by electrophoresis to determine which forms of growth hormone were secreted by the explants. All three explants secreted principally the major form of growth hormone. One adenoma however secreted unusually large amounts of two slowly migrating forms which were in low concentration in the GH-containing granules of normal pituitaries. On the other hand the adenoma produced a much lower amount of a 20,000-dalton variant than was seen in normal pituitary glands. The other two adenomas did not secrete the relatively large amounts of the slowly migrating forms although the quantity of the 20,000-dalton form was less than the amount seen in the normal pituitary. There was no evidence of proteolytic alteration of the growth hormone during the culturing process. In companion experiments with rat pituitary glands the growth hormone did not undergo proteolytic degradation in the gland left in situ for 96 h at 5 degrees C.