To assess the caloric and cardiovascular effects of dietary obesity on an important animal model of hypertension research, 60-day-old male spontaneously hypertensive (SHR) rats were fed either Agway pelleted chow (Pellet), a diet enriched in fat and sucrose (HF/M), or alternated (Cycled) between 2-week periods of HF/M diet and Agway pelleted chow. After 14 weeks, HF/M feeding elevated the body weight of SHR rats by 15% over the body weight of pellet-fed control rats. This relative obesity was accompanied by heavier white (retroperitoneal) and brown (interscapular) fat pads, heavier heart weights, and tachycardia. Paradoxically, the systolic blood pressure levels of SHR rats fed the HF/M diet were reduced. Ruled out as an explanation for the blood pressure reduction was the relative polyunsaturated to saturated fat (P/S) rati of the diet. The hypertension of SHR rats may be due in part to the effects of an underlying metabolic problem that is counteracted by the effects of HF/M feeding; HF/M feeding not only elevated body weight but also reversed the hypoinsulinemia found in pellet-fed SHR rats. The Cycled group evidenced large up-and-down swings in caloric intake that coincided with HF/M and Pellet feeding; this produced modest staircase-like changes in body weight gain. Similar to the HF/M group, the systolic blood pressure level of the Cycled group was also reduced in comparison to the blood pressure level of the Pellet group. However, the Cycled group was found to be more responsive to the pressor effects of intravenous phenylephrine, an alpha-adrenergic agonist.(ABSTRACT TRUNCATED AT 250 WORDS)