To gain a better understanding of the relationship between the female rat reproductive system and preproenkephalin (PPE) expressing neurons under physiological conditions, we examined changes in PPE mRNA levels in the mediobasal hypothalamus during the rat estrous cycle by means of northern blotting and in situ hybridization histochemistry (ISHH). In the Northern blot studies, we found that PPE mRNA levels in the mediobasal hypothalamus were significantly increased by noon of proestrus compared to those in the morning and stayed high until diestrus day 1, and returned toward low levels on diestrous day 2. In contrast, measured as controls, glyceraldehyde-3-phosphate-dehydrogenase mRNA levels were significantly higher on proestrus regardless of time of day compared to diestrus day 2, and levels of calcineurin mRNA on proestrous and estrous were significantly lower than diestrous day 1 and day 2. ISHH studies revealed that these changes in PPE mRNA levels were specific in the ventromedial hypothalamic nucleus pars ventrolateralis (VMHVL), since we could not see any significant changes in signal in other parts including ventromedial hypothalamic nucleus pars dorsomedialis and arcuate hypothalamic nucleus. In the VMHVL, PPE mRNA levels in the afternoon of proestrous were significantly higher than those in the afternoon of diestrous day 2 whereas no significant change in PPE mRNA was observed in the caudate-putamen. The present study provides additional information relevant to possible implications of PPE gene expression in female reproductive systems, since changes in PPE mRNA levels may be associated with estrogen as well as progesterone or other hormonal concentrations during the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)