ANG II increases fluid absorption in proximal tubules from young rats more than those from adult rats. ANG II increases fluid absorption in the proximal nephron, in part, via activation of protein kinase C (PKC). However, it is unclear how age-related changes in ANG II-induced stimulation of the PKC cascade differ as an animal matures. We hypothesized that the response of the proximal nephron to ANG II decreases as rats mature due to a reduction in the amount and activation of PKC rather than a decrease in the number or affinity of ANG II receptors. Because PKC translocates from the cytosol to the membrane when activated, we first measured PKC activity in the soluble and particulate fractions of proximal tubule homogenates exposed to vehicle or 10¹⁰ M ANG II from young (26 ± 1 days old) and adult rats (54 ± 1 days old). ANG II increased PKC activity to the same extent in homogenates from young rats (from 0.119 ± 0.017 to 0.146 ± 0.015 U/mg protein) (P < 0.01) and adult rats (from 0.123 ± 0.020 to 0.156 ± 0.023 U/mg protein) (P < 0.01). Total PKC activity did not differ between groups (0.166 ± 0.018 vs. 0.181 ± 0.023). We next investigated whether activation of the -, -, and -PKC isoforms differed by Western blot. In homogenates from young rats, ANG II significantly increased activated PKC- from 40.2 ± 6.5 to 60.2 ± 9.5 arbitrary units (AU) (P < 0.01) but had no effect in adult rats (46.1 ± 5.1 vs. 48.5 ± 8.2 AU). Similarly, ANG II increased activated PKC- in proximal tubules from young rats from 47.9 ± 13.2 to 65.6 ± 16.7 AU (P < 0.01) but caused no change in adult rats. Activated PKC-, however, increased significantly in homogenates from both age groups. Specifically, activated PKC- increased from 8.6 ± 1.4 to 12.2 ± 2.1 AU (P < 0.01) in homogenates from nine young rats and from 19.0 ± 5.5 to 25.1 ± 7.1 AU (P < 0.01) in homogenates from 12 adult rats. ANG II did not alter the amount of soluble PKC-, -, and - significantly. The total amount of PKC- and - did not differ between homogenates from young and adult rats, whereas the total amount of PKC- was 59.7 ± 10.7 and 144.9 ± 41.8 AU taken from young and adult rats, respectively (P < 0.05). Maximum specific binding and affinity of ANG II receptors were not significantly different between young and adult rats. We concluded that the primary PKC isoform activated by ANG II changes during maturation.