In vitro studies with human pituitary adenomas are limited by the small amount of tissue obtained, which may be contaminated by the surrounding normal pituitary tissue. In this study we investigated if the passage of adenoma tissue via thymusless nude mice could solve some of these problems. The secretory capacity of the transplanted human pituitary adenomas was demonstrated by the presence of hGH and/or hPRL in the plasma of the host mouse, while other human pituitary hormones (TSH, LH and FSH) were undetectable. The transplants, however, decreased in size with time although histologically viable adenoma tissue was recovered that resembled the original tumor. Upon trypsinization of the small tissue fragments no viable adenoma cells could be obtained. In contrast, an experimental malignant rat pituitary tumor grew steadily with time, resulting in high levels of rPRL in the nude mice recipients. Large numbers of viable tumor cells were recovered from these tumors. Thus, human pituitary adenomas transplanted in nude mice continue to release hormone(s), but the transplants decrease in size and cannot be used to isolate dispersed tumor cells for in vitro studies.