Male hypophysectomized rats in the size range of 90 to 110 g were implanted with varying numbers of spargana of Spirometra mansonoides to determine the number of worms producing maximum stimulation of growth. There occurred an increasing response up to 40 worms per rat. Beyond that number there was no further gain and probably a falling off of the effect. Forty worms gave about 65% more stimulation than 5 (115 g gain in 4 weeks versus 70 g). While most rats stopped growing at or before 6 weeks, about 25% continued, several for as long as 50 weeks or more before leveling off. Some rats increased their starting weight by a factor of 2 to 5 times, and two by 6 and 7 times, respectively. Why some rats continue to grow beyond the 6-week period, while most stop at or before that time, is not understood. Parasite-induced weight gain in mice and hamsters infected with small numbers of spargana of Spirometra mansonoides was first reported by Mueller (1963, 1965). In hamsters it was clear that true skeletal growth was involved and not mere obesity as was at first thought to be the case. Since gain was of gradual onset, investigation of this phenomenon was hindered by the length of time required to read the experiments. In 1968, Mueller and Reed reported rapid acceleration of growth in propylthiouraciltreated (PTU) rats; and Mueller (1968) extended this observation to thyroidectomized and hypophysectomized rats implanted with small numbers of spargana. Either of these systems can be read within a week or less from starting date. Thus a practical assay became available for study of the sparganum growth factor (SGF). In view of the increasing use of the hypophysectomized rat-S. mansonoides system in endocrinological and physiological studies (Steelman, Morgan, Cuccaro, and Glitzer, 1970; Steelman, Glitzer, Ostlind, and Mueller, 1971; Daughaday, 1971a, b; Steelman and Morgan, 1972; Garland and Daughaday, 1972), it becomes desirable to determine the limits and general characteristics of the parasite-induced growth effect, toward the end of standardizing the inoculum of worms. Mueller (1970) showed Received for publication 23 February 1972. * This investigation was aided by a USPHS, NIH continuing grant, 2 RO1 AI 01876-13, 14, 15. that as little as a single scolex (8 mg wet weight) was sufficient to stimulate growth in a 100-g hypophysectomized male rat, the response beginning within 2 or 3 days of the time of implanting the worms. With two and three scoleces there occurred an increasing acceleration, while with four or five there was little further gain. Thus with as little as three scoleces it appeared that maximum stimulation was already being approached. The purpose of the present study was to extend observations to larger numbers of worms per rat, to determine whether any further acceleration occurs. MATERIAL AND METHODS Hypophysectomized "90-gram" male rats were purchased from Hormone Assay Laboratories Inc., Chicago, Illinois, and kept in large plastic cages, several rats to a cage. As the rats grew the number of rats per cage was reduced in order to give the animals adequate room. The rats were individually numbered with an ear punch, and provided with food and water ad lib. Food oonsisted of a pelleted ration (Agway, Big Red), and also a canned soft food (Purina Flavor Plus, Chicken and Chicken By-Products, supplemented with vitamins and minerals, Ralston Purina Co., St. Louis, Missouri). This is a dog food, but was preferred by the rats to the pellets, and constituted practically their total solid diet. Water consisted of 10% sucrose solution. The animals were kept on a bedding of wood shavings, mixed with Pel-E-Cel, a laboratory animal waste absorbent (Paxton Processing Co., Inc., Paxton, Illinois). Cage covers were removed and the rats permitted the run of the laboratory bench for an hour or so each day in order to exercise. They rapidly became tame and made no effort to escape.