White horses are subject to age-dependent coat depigmentation. They are dark gray or black at birth and lose their coloring between their second and fourth year. Beginning at about age 10 their coat takes on a characteristic silver-gray coloring. The purpose of this paper was to find out to what extent the endogenic alpha-MSH level changes with the change in pigmentation. alpha-MSH plasma levels were determined by radioimmunologic analysis in 3 age groups of white Camarque horses: age group 1 consisted of dark horses with a mean age of 1.2 years and a mean alpha-MSH level of 106.4 pg/ml +/- 18.2, age group 2 consisted of gray horses with a mean age of 7.5 years and with a mean alpha-MSH level of 73.6 pg/ml +/- 4.8, and age group 3 consisted of silver-gray horses with a mean age of 13.5 years and a mean alpha-MSH level of 65.0 pg/ml +/- 5.3. Highly significant differences (p less than 0.001) were found between the means of age group 1 and age group 2 and between the means of age group 1 and age group 3. Determination of the ACTH plasma levels in this breed of horses showed no statistically significant differences between the various age groups. Determination of alpha-MSH and ACTH levels in a control group (n = 56) of other breeds of horses (10 black, 28 brown, and 18 sorrel) resulted in no significant differences for either hormone with regard to age or coat color. On the basis of these results it may be concluded that the degree of coat pigmentation in white Camarque horses correlates directly with the alpha-MSH plasma level.