Experiments were carried out to investigate the nature of the calcium homeostatic mechanisms in neoplastic GH3 rat pituitary cells. GH3 cells grown and maintained in Ham's F10 culture medium contained 35 nmoles calcium/mg cell protein. When stimulated by thyrotropin releasing hormone (TRH) or elevated K+ concentrations, only the latter caused cell calcium levels to rise although both resulted in hormone release. When exposed to EGTA, the GH3 cells lost calcium. When the temperature was lowered to 4 degrees C, the cells gained calcium and when rewarmed were able to extrude the previously accumulated calcium. The increased cell calcium following cold exposure could be blocked by prior treatment with rotenone. If rotenone was added subsequent to the cold exposure, it did not block the extrusion seen upon rewarming. In the absence of glucose in the medium, the GH3 cells took up more calcium upon exposure to 4 degrees C, and upon rewarming the cells could not return to their previous low levels. There are thus significant differences in calcium homeostasis between the neoplastic GH3 cells and their normal pituitary counterparts. When intracellular calcium was localized with the potassium pyroantimonate technique, there was calcium found in/on mitochondria, membrane bound vesicles and plasma membrane. Nuclear staining was sparse, and nucleolar staining was virtually absent. Upon stimulation with TRH, there was a decrease in mitochondrial calcium along with increases in both plasma membrane and nucleolar calcium levels. Since total calcium is unchanged, this indicates a significant calcium redistribution in response to TRH. The increased nucleolar calcium may reflect a calcium dependent increase in mRNA synthesis as has been reported. Since TRH presumably acts at a surface receptor, the increased plasma membrane calcium might be functionally related to receptor activation.