A radioimmunoassay for measurement of midmolecule parathyroid hormone (PTH) concentration in serum from dogs was validated for use on serum from cats. The assay detected an increase in serum concentration of PTH after IV infusion of Na2 EDTA in healthy cats. Infusion of calcium chloride caused a decrease in measured PTH. Accuracy of the assay was demonstrated by quantitative recovery of a feline parathyroid gland extract added to pooled feline sera. Mean interassay and intra-assay coefficients of variation were 0.13 and 0.07, respectively. Sensitivity of the assay was 0.1 ng of PTH/ml. The median PTH concentration measured in 40 adult cats was 3.5 ng/ml, with a range of 1.16 to 11.0 ng/ml.