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The alteration of blood pressure in Spontaneously Hypertensive Rats (Okamoto and Aoki, hereafter abbreviated as SHR) was observed under various kinds of stresses and their hypertensive vascular lesions were histologically examined. 1) Effect of immobilization Seventy to 90 day-old SIR (male 38, female 16) and age-matched normotensive rats of Wistar strain (male 33, female 5, hereafter abbreviated as NR) were restrained supinely on a board for 2-10 hours daily, 100-130 hours in total during 20-30 days. a) Stress-loaded SIR showed a highly significant increase (p< 0.001) in blood pressure (male 29±16, female 29±11 mmHg) at the end of a series of stress-loadings in comparison with either control SIR (male 11±8, female 10±8 mmHg) or control NR (male 8±6, female 5±3 mmHg), and maintained it even one month after the final stress, while a slight elevation of blood pressure (11±8 mmHg) was observed only at the end of the stress-loadings in male NR. b) Stress-loaded SIR developed severe hypertension (over 210mmHg) during or after the stress-loadings in 8 out of 38 males, and pathological studies made on them revealed cerebral hemorrhage, abdominal hemorrhage, retinal bleeding, malignant and benign nephrosclerosis, periarteritis nodosa and angionecrosis in the various organs, some of which corresponded to the lesions in malignant hypertension. 2) Effect of combined visual, auditory and electric stimuli SIR and NR were loaded with a combined stress of high pitched buzzer (duration 3-7 sec., 5-6 times per min.), flickering of incandescent lamp (100V, 100W, 40-60 times per min.) and electric shock (20-40 V Alternating current of 60Hz, 0, 5-1 sec., 5-6 times per min.) for 1-4 hours daily from the 40th day up to the 18th week after birth. a) Stress-loaded NR (male 5, 141±7mmHg) showed a significantly (p<0.05) higher blood pressure (by 11-15 mmHg) than control NR (male 10, 126±7 mmHg) after the 6th week following the onset of the stress-loadings. b) Rise in blood pressure in stress-loaded SIR (male 4, 205 ±8 mmHg) was significantly (p< 0.01) greater than that in control SIR (male 4, 175±5 mmHg), and the increment of blood pressure ( 17-31 mmHg) in the former compared with the latter was evidently larger in comparison with that (11-15mmHg) of stress-loaded NR compared with control NR. 3) Effect of chronic cold exposure Ten SIR and 1 5 NR were exposed to cold temperature (2-10°C) from 3 to 4 months after birth for 7 weeks. In the 3rd week and 7th week of cold exposure the exposed SIR showed significantly (p<0.05, p<0.01) higher blood pressure (186±16, 207±15 mmHg) than nonexposed SIR (171±13, 182±21 mmHg), respectively, while no difference was noted between the exposed NR and nonexposed 13 NR. 4) These results provided an evidence that some kinds of chronic stress-loadings augment hypertension and aggravated hypertensive lesions in SIR and it is speculated that stress might activate the causative mechanisms of the spontaneous hypertension hypothetically supposed in the hypothalamo-medullary-autonomic nervous system and the hypothalamo-adeno-hypophyseo-adrenocortical and -thyroidal systems.