The effect of uremic plasma ultrafiltrate on calcitriol synthesis was investigated. Renal 1 alpha-hydroxylase activity was measured in normal rats infused for 20 h with 20 mL of normal or uremic plasma ultrafiltrate. Renal 1 alpha-hydroxylase activity was determined by the generation of calcitriol measured 5, 10, 20, and 30 min after the reaction was initiated by the addition of cold 25(OH)D3. The activity was significantly lower in rats infused with uremic plasma ultrafiltrate. Kidney homogenates preincubated for 3 h with uremic plasma ultrafiltrate also had significantly lower renal 1 alpha-hydroxylase activity than did those preincubated with normal plasma ultrafiltrate. In addition, the effect of the putative uremic toxin, guanidinosuccinic acid (GSA), on renal 1 alpha-hydroxylase activity was studied. Normal rats infused for 20 h with 20 mL of saline solution containing 1.5 mg/dL of GSA had significantly lower renal 1 alpha-hydroxylase activity than did rats infused with normal saline. The enzyme activity was also lower in kidney homogenates preincubated for 3 h with 4 mg/dL of GSA. Enzyme kinetic analysis revealed that the inhibition of renal 1 alpha-hydroxylase by GSA was noncompetitive. It was concluded that uremic plasma contains substances that directly inhibit renal 1 alpha-hydroxylase activity.