The effect of oral doses of 300 microgram of clonidine hydrochloride on blood pressure, sedation and saliva production in 5 essential hypertensives were qualitatively similar to the effects in normotensive subjects. Peak plasma clonidine concentration (1.34 +/- 0.28 ng/ml) and plasma half-life (10.0 +/- 0.8 h) were similar to normotensives. During chronic oral dosing there was no evidence of drug accumulation. Some tolerance to the sedative and salivary flow effects occurred but no tolerance to the hypotensive effect was observed. There was a linear relationship between reduction in saliva flow and plasma levels of clonidine. The hypotensive effect was also related to plasma level at low concentrations. At plasma levels greater than 1.5 ng/ml the hypotensive effect was diminished. This loss of effect at high plasma concentration may be related to the peripheral, post-synaptic alpha-adrenoceptor agonist action of the drug.