1. A highly specific and sensitive assay based on stable isotope dilution and mass fragmentography has been developed to measure clonidine in plasma.
2. Plasma half-lives of clonidine ranged from 6·9 to 9·4 h in five normotensive subjects.
3. Plasma clonidine concentration correlated with reduction in salivary flow and degree of sedation but not with change in blood pressure.