Concentrations of LH and FSH are known to increase during normal pubertal development, but changes in the isoforms of the gonadotropins at this time have not been investigated in depth. We examined the median charge of serum LH and FSH using agarose suspension electrophoresis in 81 normal children at pubertal stages I-V. In pubertal girls there were no significant (P > 0.05) differences in the median charge of LH, but there was a small (P = 0.05) shift to more acidic FSH isoforms between pubertal stages I and IV. In boys there was a significant (P < 0.01) shift to more acidic isoforms for both LH and FSH by pubertal stage II. Further changes were not found later in puberty. Except for LH at pubertal stage I, where the median charge was similar (P > 0.05) for both sexes, the median charge was more basic (P < 0.001) for both LH and FSH in girls compared with boys at all five pubertal stages. The degree of charge heterogeneity of FSH, estimated as the peak width at half the peak height, was significantly (P < 0.01) larger at pubertal stage I than at pubertal stages III-V in both boys and girls. The charge heterogeneity of LH was similar for all pubertal stages in both sexes. In conclusion, there were few qualitative changes in the gonadotropins during normal female puberty, whereas in the male there was a dramatic shift to more acidic isoforms of LH and FSH early in puberty. This information may assist our understanding of normal and pathological processes during puberty and may be of clinical relevance in detecting the initiation of puberty in boys.