1 The isometric maximal responses ofthe guinea-pig ileum to acetylcholine and to histamine (but not those to prostaglandin E2 and to high K+) exhibited a secondary transient increase in tonus during the tonic component of the contraction. 2 After desensitizing treatment with acetylcholine or histamine, the isometric responses to either agonist showed decreased phasic and enhanced tonic components, whereas both components of the response to prostaglandin E2 were markedly depressed. 3 During the desensitizing treatment the degree of desensitization went through a maximum that coincided with the occurrence of the secondary tonic increment. 4 In low-Na+ medium, or in ouabain-treated tissues, the responses to the three agonists were similar to the respective responses in the desensitized state. 5 It is concluded that the non-specific desensitization is due to changes in Na+ translocation and that the increased tonic component ofthe isometric response is due to a reduced NaC gradient across the cell membrane and consequent increase in Ca24 loading.