The current study aims to ascertain the fate of the melanocyte stimulating hormone (MSH) receptor and its ligand [Nle4, D-Phe7] alpha-MSH (NDP-MSH) following binding to murine B16 melanoma cells. Cells were incubated with [125I]-NDP-MSH for up to 180 min and binding, internalization and degradation determined. Intracellular trafficking of the radiolabel was assessed using Percoll density gradient centrifugation of homogenized cells. Receptor down-regulation and receptor mRNA levels were also measured over 96 hr after exposure to 1 microM ligand. NDP-MSH accumulation increased with time in a temperature-dependent manner and was inhibited by excess peptide. The ligand was rapidly internalized and translocated to the lysosomal compartment where it was degraded. Internalization was accompanied by a loss or down-regulation of cell surface receptors, suggesting internalization of the NDP-MSH-receptor complex. No recycling of the receptors between the plasma membrane and intracellular compartments could be detected in this cell-line. Approximately 15% of the surface receptors were resistant to down-regulation, possibly indicating receptor heterogeneity. Down-regulation persisted for up to 96 hr and was accompanied by a decrease in MSH receptor mRNA levels 48 hr after treatment. However, before this time, transcript levels were the same in treated and control cells. In contrast to what was seen with NDP-MSH, cell surface receptors removed with trypsin were rapidly replaced. These results show that NDP-MSH not only induced MSH receptor internalization but also inhibited receptor turnover, resulting in a prolonged down-regulation. It is concluded that, in B16 cells, the MSH receptor undergoes ligand-dependent internalization, resulting in a prolonged down-regulation.