Microinjection of increasing doses of ATP (0.31, 0.62, 1.25 and 2.5 nmol/50 nl) into the nucleus tractus solitarii (NTS) produced a dose-dependent pressor response. Prazosin abolished the pressor response and produced no change in the bradycardic response to ATP. Microinjection of PPADS (0.25 nmol/50 nl), a non-selective P2 receptor antagonist into the NTS, reduced the bradycardic response but had no effect on the pressor response to microinjection of ATP (1.25 nmol/50 nl) into the NTS. Microinjection of suramin (2 nmol/50 nl), another non-selective P₂ receptor antagonist, had no effect on the pressor and bradycardic responses to microinjection of ATP (1.25 nmol/50 nl) into the NTS. Antagonism of A₁ receptors of adenosine with DPCPX also produced no changes in the cardiovascular responses to microinjection of ATP into the NTS. The involvement of excitatory amino acid (EAA) receptors in the pressor and bradycardic responses to microinjection of ATP into the NTS was also evaluated. Microinjection of kynurenic acid, a non-selective EAA receptor antagonist (10 nmol/50 nl), into the NTS reduced the bradycardic response and had no effect on the pressor response to microinjection of ATP into the NTS. The data show that a) microinjection of ATP into the NTS of awake rats produced pressor and bradycardic responses by independent mechanisms, b) the activation of parasympathetic component may involve an interaction of P₂ and EAA receptors in the NTS and c) the sympathoexcitatory response to microinjection of ATP into the NTS was not affected by the blockade of P₂, A1 or EAA receptors.