Lesions of the medial preoptic area (MPOA) result in the robust display of steroid-induced lordosis by otherwise unresponsive, immature guinea pigs. This study was designed to test the hypothesis that destruction of this region also hastens the onset of ovarian cyclicity, and to ascertain whether lesions of the septum produce effects on puberty similar to those observed following MPOA lesions. Juvenile (13-15 days old), ovariectomized or ovary-intact guinea pigs received bilateral electrolytic or sham lesions of the MPOA or septum. Bilateral lesions of the septum did not influence the display of lordosis or any aspect of estrous cyclicity. However, a higher percentage of guinea pigs with MPOA lesions than with sham lesions displayed progesterone-facilitated lordosis at 22, 29, and 39 days of age; by 49 days of age, the two groups responded in a similar, robust fashion. Females with MPOA lesions were significantly heavier and older than sham-lesioned or nonmanipulated females on the day of first vaginal opening. These data suggest that activity in the MPOA tonically inhibits the display of progesterone-facilitated lordosis through at least 39 days of age but does not suppress the onset of ovarian cyclicity in guinea pigs. Contrary to our hypothesis and in contrast to what has been observed in rats, signals originating in or passing through the MPOA may be required in order for estrous cyclicity to be initiated at the appropriate age. Furthermore, neither of these effects of MPOA lesions appears to be due to damaging efferent or afferent projections through the septum.