We examined the contractile effect of angiotensin II on isolated rat glomeruli and cultured glomerular mesangial cells, as well as the possible involvement of calcium in this contraction. In addition, the effect of rat atrial natriuretic peptide (ANP) on angiotensin II-induced contraction was studied. Computer-assisted image analysis was used to measure glomerular cross-sectional area and planar mesangial cell surface area in several experimental conditions. Angiotensin II induced a decrease in glomerular cross-sectional area and planar mesangial cell surface area that was both time and dose dependent. This effect was partially blocked by preincubation with verapamil or TMB-8, the latter compound being more effective. Incubation with ANP prevented angiotensin II-induced reduction of glomerular cross-sectional area and planar mesangial cell surface area and reversed contraction in mesangial cells. Incubation with TMB-8 impaired the effect of ANP. These data suggest that angiotensin II could exert part of its physiologic effects in the kidney through calcium-dependent mesangial and glomerular contraction. ANP inhibited this contraction, possibly explaining some of its renal effects.