Prepubertal (immature) and mature female rabbits were actively immunized against testosterone-3-oxime-bovine serum albumin over a period of 11 weeks. The antibody titre was significant in all animals by 5 weeks. The concentration of FSH in prepubertal animals decreased significantly (
P< 0·001) between weeks 1 and 5, but no significant changes were observed in the concentration of LH at any time. After immunization for 8 weeks, there was a significant ( P< 0·05) increase in the serum concentration of androgen and the percentage of bound testosterone also increased ( P< 0·05). The serum concentration of oestradiol increased after immunization for 11 weeks, compared with values at 8 weeks ( P<0·05) and oestradiol binding also rose by week 5 ( P<0·01). Libido was not affected and significantly ( P< 0·005) increased numbers of ovulations were noted in immunized animals. These results suggest that immunization of the female rabbit against testosterone may disrupt the normal regulation of follicular maturation.