1. Experiments were conducted on renal cortical slices from sixty-one adult mongrel dogs in order to evaluate the relationship between p-aminohippurate (PAH) transport and calcium. Tissues were found to be viable for at least 180 min.2. PAH uptake and the slice/medium (S/M) concentration ratio increased in normal tissues from 3 to 90 min whether data were expressed on a tissue weight or protein content basis. PAH uptake increased with time in calcium-depleted tissues, but all values were lower than normal. At 90 min, PAH S/M in calcium-depleted tissues (4.20 +/- 0.18) was reduced 40% compared to normal tissues (6.90 +/- 0.23).3. Over the time period of 3-90 min incubation, protein concentration ranged from 89.4 to 99.4 mg/g cortex in calcium-depleted tissues. During the same time period in normal tissues, protein concentration ranged from 105 to 117.6 mg/g cortex. No change was observed in medium protein concentration of calcium-depleted tissues.4. Lineweaver-Burke analysis showed similar values for K(m) in normal and calcium-depleted tissues (0.45-0.50 mumole ml.(-1)) although V(max) fell from control values of 0.082 to 0.052 mumole g(-1) min(-1) during calcium depletion.5. After 90 min incubation, addition of EGTA (7, 14 or 21 x 10(-3)M) to media containing calcium was associated with decreased S/M and uptake rates compared to control values from the same animals. The decreases observed were similar to those seen in tissues incubated in calcium-free media.6. Increased cell volume in the presence of calcium was not associated with decreased PAH uptake. Removal of calcium in association with constant cell volume was associated with reduced PAH uptake. Replacement of calcium, but not magnesium, is associated with a return of uptake to normal rates.7. It is concluded that (a) calcium removal causes increased water uptake into cells, (b) calcium removal causes a reduced V(max), (c) calcium is required for maintenance of intracellular PAH, (d) the effect of calcium on PAH is specific and reversible, and (e) the effect of calcium is not mediated by volume changes.