Melanotan one and Melanotan II peptides allow your body's own tanning system to create a tan (by increasing natural melanin production). This allows individuals who ordinarily would not tan and just burn (or who tan with difficulty) to actually be able to tan easily when getting limited UV exposure. There is a company out of Australia called "Clinuvel" which is developing the first version of melanotan (under the name "afamelanotide") to treat symptoms of a series of skin diseases that respond favorably to folks' skin becoming tan (like polymorphous light eruption (PMLE) and actinic keratosis (AK)). Melanotan-II has the additional effect of causing spontaneous sexual arousal (increasing one's libido) in both men and women.
A more technical explanation]Melanotan peptides are man made (synthetic) forms of a naturally occurring hormone called alpha melanocyte stimulating hormone (a-MSH). This hormone stimulates cells called melanocytes. Melanocytes lie in the deeper layers of the outer layer of the skin (epidermis).
Melanocytes make the pigment 'melanin' when the skin is exposed to sunlight. The melanin then moves into surrounding skin cells to help protect them against the sun's rays. Melanin makes fair skinned people tan in the sun. Melanocytes are more active in darker skinned people and make more melanin. So dark skinned people have more protection against skin cancer than fair skinned people.