Evidence that endogenous progesterone (PROG) is neuroprotective after traumatic brain injury (TBI) is supported by the findings that pseudopregnant female rats present less edema and achieve better functional recovery than do male rats. PROG in the nervous system may originate from steroidogenic glands or can be locally synthesized. 3beta-Hydroxysteroid dehydrogenase/5-ene-4-ene isomerase (3beta-HSD) is the key enzyme in the biosynthesis of PROG. In the present study, we investigated the effects of pseudopregnancy and TBI on brain 3beta-HSD mRNA expression and on PROG levels. Twenty-four hours after bilateral contusion of the medial prefrontal cortex of rats, 3beta-HSD mRNA expression was analyzed by in situ hybridization while PROG levels were measured by gas chromatography/mass spectrometry. Similar levels of 3beta-HSD mRNA expression were observed in males and pseudopregnant females in the non-injured groups. At this time point, there was a significant decrease in the 3beta-HSD mRNA expression in the contusion site within the frontal cortex in both males and pseudopregnant females. In all other regions analyzed, 3beta-HSD mRNA expression was not affected by TBI and there was no difference between males and pseudopregnant females. The high decrease in the expression of the 3beta-HSD mRNA in the lesion site 24 h after TBI suggests a possible decrease in locally synthesized PROG in lesion site without change in the other brain regions. This decrease has less impact in pseudopregnant females since they have high plasmatic and brain levels of PROG compared to males.