α-Melanocyte-stimulating hormone (α-MSH) and adrenocorticotropin (ACTH) immunoreactivity (IR) was measured in the blood of 22 healthy women with normal ovulatory process in the early and late follicular (near to ovulation) phases and in the early luteal phase of the menstrual cycle. Plasma α-MSH IR ranged from undetectable values to 81.3 pg/ml, the highest levels being found in the late follicular phase (15.52 ± 4.16 pg/ml). In contrast, plasma ACTH IR was always detectable (range: 18.5–63.2 pg/ml), but its concentration did not differ significantly between the 3 phases of the menstrual cycle. High-pressure liquid chromatography fractionation of Sep pak C18-purified α-MSH IR revealed in all 3 phases the presence of 3 major peaks of α-MSH IR, coeluting with desacetyl-α-MSH, α-MSH and diacetyl-α-MSH, respectively. The most abundant peak always coeluted with authentic desacetyl-α-MSH, and the ratio between this deacetylated and the other 2 acetylated forms was similar in the 2 follicular phases (1:1.25 and 1:1.16 in the early and late phase, respectively), but significantly different in the luteal phase (1:0.48). The fluctuations in plasma concentration of the above MSH-related peptides suggest that different rates of α-MSH acetylation and release take place in the pituitary gland depending on the phase of the menstrual cycle.