The time course of toxicity in a 28 year old man following a 100 mg accidental overdose of clonidine hydrochloride is compared with the decline of plasma clonidine over 5 days. The concentration data were analyzed by nonlinear least squares regression, and fitted to a model which dissociated the blood pressure effects into pressor and depressor components. According to this analysis, there appeared to be two phases of toxicity over time--a hypertensive, and a hypotensive phase. The hypotensive and the hypertensive phases may result primarily through differential stimulation of central alpha 2-, alpha 1-, and vascular post-synaptic alpha 2-adrenoceptors as plasma concentrations fall.