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We have attempted to characterize the as yet unspecified circulating factor(s) in spontaneously hypertensive rat (SHR) serum that elevate(s) the cytoplasmic calcium ion concentration in vascular smooth muscle cells (VSMC). We measured the cytoplasmic calcium ion concentration in cultured VSMC before and after incubation with serum from SHR or Wistar–Kyoto rats (WKY). Incubation with serum from overtly hypertensive 8-week-old SHR, but not with serum from 8-week-old WKY, significantly elevated the cytoplasmic calcium ion concentration in VSMC from 8-week-old WKY and 4-week-old SHR in a dose- and an incubation period-dependent manner, although no significant elevation occurred in VSMC from 4-week-old WKY. Neither the heat-immobilized nor the trypsinized 8-week-old SHR serum elevated the cytoplasmic calcium ion concentration. Ultrafiltration procedures indicated that the molecular weight of the factor should be 10 000-30 000 daltons. Incubation with serum from 4-week-old SHR or WKY did not elevate the cytoplasmic calcium ion concentration. These results strongly suggest that: (1) a cytoplasmic calcium ion concentration elevating factor exists in serum from 8-week-old SHR but not from 4-week-old SHR, or 4- or 8-week-old WKY; (2) this factor should be of a protein nature, with a molecular weight of 10 000–30 000 daltons; (3) responsiveness to the factor appears in VSMC at a younger age in SHR than in WKY; (4) the factor elevates the cytoplasmic calcium ion concentration to the level determined a priori not by the strain but by the age of the rats from which the VSMC originate; and (5) the a priori determined level increases with the age of the rats.