Animals at advanced ages exhibit a reduction in central leptin sensitivity. However, changes in growth, metabolism and obesity risk occur much earlier in life, particularly during the transition from youth to middle age. To determine when initial decreases in central leptin sensitivity occur, leptin-dependent suppression of food intake was tested in 8, 12 and 20 week old male, chow fed Sprague Dawley rats. Intracerebroventricular (icv) leptin injection (3 ug) suppressed 24 h food intake in 8 and 12-week old rats (P < 0.05), but not in 20-week old rats. To identify potential cellular mediators of this resistance, we focused on protein tyrosine phosphatase 1B (PTP1B), a recently described inhibitor of leptin signaling. PTP1B protein levels, as determined by Western blot, were significantly higher in mediobasal hypothalamic punches collected from 20-week old rats compared with 8-week old rats (P < 0.05). When 20-week old rats were fasted for 24 h, levels of hypothalamic PTP1B decreased (P < 0.05) coincident with a restoration of leptin sensitivity. To directly test whether inhibition of PTP1B restores leptin sensitivity, 20-week old chow-fed rats were pretreated with a pharmacological PTP1B inhibitor 1 h before leptin, and 24 h food intake was recorded. As expected, leptin alone produced a small but non-significant reduction in food intake. However, pretreatment with the PTP1B inhibitor resulted in a marked improvement in leptin-dependent suppression of food intake (P < 0.05). These data are consistent with the hypothesis that increases in PTP1B contribute to hypothalamic leptin resistance as rats transition into middle-age.