Preweanling rats were tested to determine whether angiotensin II (ANG II) and aldosterone (Aldo) act synergistically to enhance salt appetite at 12 and 17 days. Twelve-day-old pups received one of four hormone treatments in four doses: 1) ANG II only [1, 2, 10, or 100 ng pulse intracerebroventricular (icv)], 2) Aldo only (1, 2, 10, or 40 micrograms/day sc), 3) Aldo + ANG II (four individual doses combined), or 4) vehicle. Seventeen-day-old rats received the same treatments in two doses (2 or 100 ng ANG II; 2 or 40 micrograms Aldo). Pups were presatiated with milk through anterior oral catheters and then given either 4% NaCl or water for 30 min. Intake was assessed by body weight change. At both ages, ANG II enhanced salt (and water) intake, and Aldo enhanced salt (but not water) intake. Minimum effective doses were comparable to those reported for adults. ANG II-Aldo synergy was absent at 12 days and present at 17 days, when salt intake was 590% greater than the summed intakes evoked by ANG II and Aldo alone. The neural mechanisms for ANG II-Aldo synergy thus mature later than those mediating the hormone's individual actions in arousing salt appetite.