We have studied the involvement of proteolytic pathways in the regulation of the Na/P i cotransporter type II by parathyroid hormone (PTH) in opossum kidney cells. Inhibition of lysosomal degradation (by leupeptin, ammonium chloride, methylamine, chloroquine, l -methionine methyl ester) prevented the PTH-mediated degradation of the transporter, whereas inhibition of the proteasomal pathway (by lactacystin) did not. Moreover it was found ( i ) that whereas lysosomal inhibitors prevented the PTH-mediated degradation of the transporter they did not prevent the PTH-mediated inhibition of the Na/P i cotransport and ( ii ) that treating opossum kidney cells with lysosomal inhibitors led to an increased expression of the transporter without any concomitant increase in the Na/P i cotransport. Further analysis by subcellular fractionation and morphological techniques showed ( i ) that the Na/P i cotransporter is constitutively transported to and degraded within late endosomes/lysosomes and ( ii ) that PTH leads to the increased degradation of the transporter in late endosomes/lysosomes.