To characterize the hypothermia-induced changes in the activity and kinetic constants for muscarinic receptors, we investigated the effects of hypothermia on the onset and offset of action of phenoxybenzamine from the muscarinic receptors of the longitudinal muscle of the guinea-pig isolated ileum. At 37 degrees C, the onset of phenoxybenzamine action was very rapid (less than 5 min) and there was no apparent recovery of the response to ACh 70 min after washing off the phenoxybenzamine. However, the onset at 24 degrees C was very slow (30 min) and there was a complete recovery of the response to ACh 40 min after washing off the unoccluded phenoxybenzamine. We concluded that the lack of reversibility of the effects of phenoxybenzamine at 37 degrees C is due to the predominance of covalent binding between the receptors and phenoxybenzamine whereas at 24 degrees C the blockade of the muscarinic receptor by phenoxybenzamine is mainly due to simple occlusion.