The injection of acetylcholine (ACh) into suspensions of normal Chinese hamster ovary (CHO) cells and CHM cells (CHO cells transfected with the M1 muscarinic receptor) results in an increased production of heat superimposed upon the on-going cellular metabolism. During this eight minute heatburst, under our experimental conditions, the CHM cells liberate one and one-half times as much heat per cell as the CHO cells which have fewer M1 muscarinic receptors. Cellular integrity is required for this thermal response to ACh since it is abolished when the cell is disrupted by trituration. Experiments in the presence of inhibitors of glycolysis indicate the ACh receptor mediated thermal response in CHM cells to have both anaerobic and aerobic glycolytic components. An intact respiratory apparatus is required for the ACh thermal response to occur. Five inhibitors binding at locations both inside and outside the cell membrane and acting at different sites along the phosphoinositide/ calcium signalling/ phosphokinase C cascade resulting from stimulation by ACh --- all markedly suppress the thermal response to ACh in the CHM cells but have little effect upon the thermal response of the CHO cells. This comparison, made within the same cell type, convincingly shows that the thermal response to ACh is different in the wild-type and transfected cells. This work shows that receptor mediated events and the role of different metabolic pathways can be studied non-invasively by monitoring cellular heat production.