The peptide hormone α-melanotropin (α-MSH) is well known to stimulate pigment dispersion in dermal melanophores of lower vertebrates 1 and pigment formation in epidermal melanocytes 2 and melanoma cells. 3 α-MSH is also thought to play an important role in a number of other systems including fetal development, 4 axonal regeneration, 5,6 and modulation of neuronal activity and behavior. 7 Detailed reviews on the biological effects of melanotropins can be found in Volume II of this series.