The claim has been made that thyrotrophin (TSH) can augment the action of growth hormone (GH) to stimulate growth of the epiphysial cartilage plate of the hypophysectomized rat's tibia. The TSH induces its effect via secretion of thyroid hormones which in turn enhance the stimulatory action of GH. If this is true then the employment of the tibia test, whose endpoint is the increase in thickness of the epiphysial cartilage plate in response to GH present either in crude pituitary extracts or relatively purified preparations, which also are likely to contain modest or appreciable quantities of TSH, requires further examination. The present study utilized various fractions of crude pituitary extracts from intact and thyroidectomized rats that respectively contained appreciable quantities of GH or essentially no GH. Fractional aliquots of pituitary extracts from thyroidectomized rats were administered concomitantly with graded doses of exogenous GH to hypophysectomized rats to determine the point at which TSH in the extracts was sufficiently able to stimulate significant tibial plate growth when compared to recipients given GH alone. Purified GH and TSH were also administered in various doses to hypophysectomized recipients in a further attempt to delineate the dose range at which TSH augments the action of GH to promote significant chondrogenesis of the epiphysial plate. The results indicate that the enhancement of the GH effect on the cartilage plate by TSH was evident only when quantities above 100 microng bovine GH were co-administered with 100 mU bovine TSH. As little as 40 mU TSH augmented the growth effect of 400 microng GH on the cartilage plate, demonstrating that smaller quantities of TSH could potentiate larger quantities of GH. These data, therefore, suggest that extracts equivalent to not more than one-half of a normal adult rat's anterior pituitary gland should be administered to hypophysectomized rats for bioassay of GH. Fractions of glands greater than this may contain sufficient amounts of TSH to augment the appreciable quantities of GH already present.