Alkaline H2O2 degradation of red hair pheomelanin gave, besides 6-(2-amino-2-carboxyethyl)-2-carboxy-4-hydroxybenzothiazole (BTCA), a new product which was identified as 7-(2-amino-2-carboxyethyl)-2-carboxy-4-hydroxybenzothiazole (BTCA-2) originating from 2-S-cysteinyldopa (2SCD) derived units. BTCA-2 was also obtained from a variety of pheomelanic tissues and synthetic pigments. Simultaneous determination of BTCA and BTCA-2 in segments of red hair locks taken at variable distances from the scalp in a group of 19 individuals indicated an abrupt drop of BTCA yields on passing from root to tip, whereas BTCA-2 values remained virtually constant throughout hair length. Analysis of 4-amino-3-hydroxyphenylalanine (AHP) and 3-aminotyrosine (AT) in the same lock segments showed a closely similar trend, whereas yields of thiazole-2,4,5-tricarboxylic acid (TTCA) increased with increasing the distance from the scalp. Prolonged exposure of hair locks to sunlight caused a significant decrease in BTCA-, but not BTCA-2-yielding elements. Finally, model studies showed a substantial degradation of 5SCD-, but not 2SCD-derived units, during pheomelanin synthesis in vitro. It is concluded that red hair pheomelanin consists of a degradable 5SCD-derived bulk component associated with stable 2SCD-derived units. Structural degradation occurs during hair growth probably as a result of oxidative processes related in part to sun exposure.