The purpose of this study was to test the hypothesis that differences in voltage-gated Ca2+channels increase with age during the development of sustained hypertension in the spontaneously hypertensive rat (SHR). Using patch-clamp methods, we measured whole-cell Ca2+currents in freshly isolated myocytes from small mesenteric arteries of juvenile (5 to 7 weeks), young (10 to 12 weeks), and mature (19 to 23 weeks) Wistar-Kyoto rats (WKY) and SHR. Indirect tail artery systolic pressure increased progressively with age in SHR (from 125 +/- 5 to 159 +/- 5 to 192 +/- 5 mm Hg) but only in the younger WKY (from 107 +/- 6 to 130 +/- 4 to 136 +/- 4 mm Hg). Peak Ca2+current density (current per cell capacitance) was larger in SHR than WKY myocytes at all ages (at 6 weeks, 3.5 +/- 0.4 versus 2.3 +/- 0.2 pA/pF; at 12 weeks, 3.8 +/- 0.2 versus 3.1 +/- 0.2; at 20 weeks, 4.9 +/- 0.4 versus 3.3 +/- 0.4). Cell capacitance (surface area) was significantly smaller in 12-week-old SHR than WKY (25.2 +/- 1.1 versus 31.8 +/- 1.6 pF), but no differences were found in the 6- or 20-week-old groups. There were significant differences in Ca2+current with strain, age, and voltage but no significant age-strain interactions. The ratio of peak Ca2+current for SHR to that of WKY declined linearly with voltage at all ages, suggesting differences in the voltage dependence of Ca2+current activation. The voltage dependence of Ca2+current was shifted to the left in SHR compared with WKY at all ages. Activation curves were shifted significantly in the negative voltage direction only in 20-week-old SHR myocytes. We have found differences with age in Ca2+current density and its voltage dependence in SHR compared with WKY during the phase of development in which blood pressure becomes established in the SHR. The net effect of these differences predicts a larger Ca2+current in SHR at voltages in the physiological range of membrane potential. (Hypertension. 1997;29:1329-1336.)