Freshly prepared extracts of rat, dog and human pituitaries filtered on Sephadex G–200 exhibited two distinct peaks of growth hormone (GH) immunoreactivity. The majority of GH had an elution vol similar to that of purified GH, but a small fraction (one percent or less of the total GH) was eluted just after the void vol. Its large molecular size was confirmed on Sepharose 4B gel filtration. No immunologic dissimilarity was observed between the two forms of the hormone. The large molecular sized GH (LGH) could not be converted into the smaller molecular sized GH (SGH) by sonication or trypsin treatment, but was converted by exposure to guanidine. Synthesis of LGH was studied in vitro in both rat pituitaries and in the GH producing rat pituitary tumor, MtT4. A much higher SA of LGH as compared to SGH was observed, indicating a more rapid turnover of LGH. Although the conversion of LGH to SGH within the pituitary could not be demonstrated, and LGH was not secreted per se, LGH was shown to be a precursor of SGH released into the medium. LGH appears to be related more directly to synthesis and/or release than to hormone storage, but its precise physiologic role remains to be defined. (Endocrinology91: 262, 1972)