Calcium-binding proteins play potentially important roles in neurogenesis and neuroprotective mechanism(s). Some evidence exists that brain calbindin-D28K (CALB) is regulated by androgens. In the present study, calretinin (CALRET) and CALB patterns were determined by Western analysis in the medial basal hypothalamus (MBH) from male rats along with assaying plasma testosterone levels during postnatal development. Testosterone levels were very low in 7-, 10-, and 30-day-old animals (approximately 0.5 ng/mL), increased in a stair-step fashion to peak levels at 90 days (approximately 3.8 ng/mL), then declined with increasing age to very low levels at 300 days of age (approximately 0.3 ng/mL). At 7 and 10 days, MBH CALRET and CALB levels were low; however, at Day 30 a significant twofold increased was observed. Thereafter, in 60-, 120-, 180-, and 300-day-old animals MBH CALRET and CALB levels were, in general, comparable to 30-day-old values. These findings suggest that there is not a clear correspondence between the androgen status in male rats and the calcium-binding proteins (CALRET & CALB) expressed in the MBH. Therefore, it appears that brain CALRET and CALB are regulated in a developmental fashion with significant increases in expression occurring around the 4th postnatal week.