Plasma aldosterone levels and renin activity were estimated together with plasma electrolyte concentrations and a few other measurements in eight calves for 14 days after birth. Plasma aldosterone stayed at relatively high levels of 155.4-184.0 pg/ml during 24 hrs after birth and significantly decreased to 88.5 pg/ml during a next 24 hr. Thereafter, the plasma aldosterone remained at low levels until 14 days after birth. Although plasma renin activity showed a slight rise at 4 hrs and a slight decline at 12 hrs after birth, the activity as a whole remained stable within 0.97-1.58 ng/ml/hr throughout the experimental period. The change in plasma aldosterone was similar to previous reports on different species, but that in plasma renin activity was different from previous reports on other species. During a 24 hrs period after birth, there was a significant positive correlation between plasma renin activity and aldosterone levels (r=0.621, P<0.001).