We investigated the effect of thyroid hormone status on renal handing of Ca2+. Further, like kinetics of Ca2+ transport across brush-border membrane (BBM) and basolateral membrane (BLM) of renal epithelial cells was carried out. FE(Ca) was decreased in hyperthyroid (Hyper-T) rats and increased in hypothyroid (Hypo-T) rats as compared to euthyroid (Eu-T) rats. Ca2+ uptake into renal brush-border membrane vesicles (BBMV) was increased in Hyper-T rats and decreased in Hypo-T rats as compared to Eu-T rats. K(m) was lower in Hyper-T rats and higher in Hypo-T rats as compared to Eu-T rats whereas, V(max) remained unaltered. The transition temperature for calcium uptake varied inversely with the thyroid hormone status. Renal BBM of Hyper-T rats showed decreased anisotropy and polarisation of DPH as compared to EU-T rats whereas these values were increased in Hypo-T rats. Thus, the altered BBM fluidity appears to modulate Ca2+ transport across BBM. Na+/Ca2+ exchange activity of renal cells was increased in Hyper-T and decreased in Hypo-T rats as compared to Eu-T rats. V(max) for Na+/Ca2+ exchange was increased in Hyper-T rats and deceased in Hypo-T rats as compared to Eu-T rats, whereas, [Na+](0.5) was similar in all three groups. The c-AMP levels of renal cortex of Hyper-T rats was increased and that of Hypo-T rats decreased as compared to Eu-T rats. Thus, thyroid hormones increased Ca2+ reabsorption in the kidney of rat. Thyroid hormone-mediated modulation of BBM fluidity appears to stimulate Ca2+ uptake into renal BBMV. Thyroid hormones possibly activated the Na+/Ca2+ exchanger through cAMP-dependent pathway.