Abstract 1. Adaptation of muscarinic receptors (MR)—muscarinic stimulation—induced intracellular Ca 2+ mobilization during short-heat exposure (33°C). 2. Heat-exposure for 48 hr decreased the carbachol (CCh)-stimulated cytosolic C 2+ concentration increase. 3. The number of MR on cell surface increased transiently at 24 hr with a subsequent decrease at 48 hr. 4. CCh-stimulated inositol trisphosphate (IP 3) formation decreased at 48 hr. 5. In saponin-permeabilized cells, 1,4,5-IP 3-induced 45Ca 2+ release decreased at 24 hr. 6. The data suggest that the adaptation for increased muscarinic stimulation occurs at IP 3 generating sites as well as at intracellular IP 3 receptor sites during heat exposure.