Supersensitivity of salivation in response to pilocarpine after withdrawal of chronically administered hyoscine in the mouse.

1. The duration of the effect of a single large dose of hyoscine, in reducing the salivary response of mice to pilocarpine, was established as less than 66 hours.2. Supersensitivity was observed, after daily oral dosing with hyoscine, in the increased salivation of mice in response to pilocarpine injected at least 66 h after withdrawing hyoscine.3. The minimum duration of pretreatment with hyoscine that resulted in supersensitivity was 5 days. The daily dose was more effective if divided.4. The period after withdrawal for which supersensitivity could be detected was 6 days.5. The maximum salivary response to pilocarpine was increased by chronic hyoscine pretreatment.6. The antagonism of a single dose of hyoscine to pilocarpine salivation, as expressed by the dose-ratio of pilocarpine, was not altered by chronic hyoscine pre-treatment.