A two-site intact immunoradiometric assay was validated for the measurement of parathyroid hormone (PTH) in the cat, by assessment of precision, sensitivity and specificity. The mean intra- and interassay coefficients of variation were 12·7 and 12·8 per cent, respectively. The sensitivity of the assay was 3·90 pgl/ml. The presence of carboxyl fragment interference was suspected from the non-parallel dilution of two of seven feline samples and the decreased recovery of PTH on addition of carboxyl terminal fragment PTH to samples. The plasma PTH concentration, measured using this assay, in 40 clinically healthy cats was 10·9 ± 5·3 pg/ml (mean ± SD). The assay demonstrated appropriate responses in cases of secondary renal hyperparathyroidism, hypercalcaemia of malignancy and iatrogenic hypoparathyroidism. Storage characteristics would permit posting of samples. This assay will therefore provide a useful non-invasive tool for the diagnosis of disorders of calcium metabolism in the cat.