The present studies were carried out to provide further evidence for the hypothesis that the hypotensive response to adenosine A3 receptor activation in the anaesthetized rat involves mediator release from mast cells. Male Sprague-Dawley rats were anaesthetized and given just supramaximal hypotensive doses of either the non-selective A3 receptor agonist, N6(-2)-(4-aminophenyl)ethyladenosine (APNEA: 100 micrograms/kg, preceded by the A1/A2 receptor antagonist, 8-p-(sulphophenyl)theophylline, to "isolate' the A3 receptor-mediated component of the response), the mast cell degranulating agent, compound 48/80 (300 micrograms/kg) or the vehicle for APNEA, intravenously. Blood was withdrawn from a carotid artery between 2 and 10 min after the injection and plasma and serum histamine concentrations measured. Samples of connective tissue (surrounding the abdominal musculature), thymus, mesenteric lymph node, kidney, skin and diaphragm were removed for histological analysis. The plasma and serum histamine concentrations were markedly and significantly higher in the APNEA- or compound 48/80-treated animals compared to vehicle-treated controls. Consistent with this, a substantially greater proportion of mast cells was seen to be undergoing degranulation in all tissues removed from animals treated with APNEA or compound 48/80 compared to those from rats treated with vehicle. Thus, adenosine A3 receptor activation results in rapid mast cell degranulation in the anaesthetized rat. The data provide further evidence of a key role for the mast cell in adenosine A3 receptor-mediated hypotension in this species.