Clearance and micropuncture experiments were performed on male Cercopithecus monkeys weighing between 2 and 4 kg to evaluate the function of the superficial nephron. The mean glomerular filtration rate was 4.47 ± 0.32 ml/min. The fraction of filtered sodium, potassium, calcium, magnesium, and phosphate remaining at the end of the accessible proximal tubule were 54 ± 3, 64 ± 4, 70 ± 3, 79 ± 5, and 39 ± 5%, respectively. The concentration of magnesium and calcium in the tubule fluid rose significantly along the length of the proximal tubule. The tubule fluid bicarbonate concentration was measured by determination of total CO2 with microcalorimetry. The tubule fluid to blood bicarbonate ratio fell to 0.48 ± 0.06 at the late proximal tubule collection site, as the tubule fluid to ultrafiltrable chloride concentration ratio rose to 1.08 ± 0.02. Thus, bicarbonate is reabsorbed in preference to chloride in the superficial proximal tubule of the monkey, to provide a potential driving force for water and salt absorption. By the time the tubular fluid reached the distal tubule sampling site, most of the filtered sodium potassium, calcium, magnesium, and chloride was reabsorbed, suggesting the loop of Henle as a major site of ion reabsorption. Segments beyond the distal tubule collection site reabsorbed little of the delivered ions. These studies indicate that the function of the superficial nephron resembles that of species previously studied, except that calcium reabsorption is demonstrably diminished in the proximal convoluted tubule relative to other