Epidemiological studies have reported statistically significant associations between the levels of ambient particulate matter (PM) and the incidence of morbidity and mortality, particularly among persons with cardiopulmonary disease. While similar effects have been demonstrated in animals, the mechanism(s) by which these effects are mediated are unresolved. To further investigate this phenomenon, the cardiovascular and thermoregulatory effects of an oil combustion-derived PM (HP-12) were examined in spontaneously hypertensive (SH) rats. The particle used in this study had considerably fewer water-soluble metals than the residual oil fly ash (ROFA) particles widely used in previous animal toxicity studies, with Zn and Ni constituting the primary water-leachable elements in HP-12. Rats were surgically implanted with radiotelemeters capable of continuously monitoring electrocardiogram (ECG), heart rate (HR), systemic arterial blood pressure (BP), and core temperature (T(co)). Animals were divided into four dose groups and were administered one of four doses of HP-12 suspended in saline vehicle (0.00, 0.83, 3.33, 8.33 mg/kg; control, low, mid, and high dose, respectively) via intratracheal instillation (IT). Telemetered rats were monitored continuously for up to 7 days post-IT, and were sacrificed 4 or 7 days post-IT. Exposures to mid- and high-dose HP-12 induced large decreases in HR (decreasing 30-120 bpm), BP (decreasing 20-30 mmHg), and T(co) (decreasing 1.2-2.6 degrees C). The decreases in HR and BP were most pronounced at night and did not return to pre-IT values until 72 and 48 h after dosing, respectively. ECG abnormalities (rhythm disturbances, bundle branch block) were observed primarily in the high-dose group. This study demonstrates substantial dose-related deficits in cardiac function in SH rats after IT exposure to a low-metal content, combustion-derived particle.