To investigate whether short-term fasting affects the pituitary-testicular axis in obese subjects, 9 massively obese men (Body Mass Index 39.0 ± 1.3, mean± sem) were given two identical iv GnRH tests, the first (control) after an overnight fast, the second after 56 h of food deprivation. Short-term fasting augmented the GnRH-induced LH incremental area by 26%(1317±251 vs 1661 ± 297 U · 1−1 · min−1, p <0.05), but failed to affect the corresponding testosterone incremental area. Eight non-obese normal men (Body Mass Index 22.2 ± 0.5) were investigated for comparison. All of them were studied according to the same protocol as the obese group. Short-term fasting increased the GnRH-elicited LH response by 67% in the non-obese group (LH incremental areas 2147 ± 304 vs 3581 ± 256, p <0.01), and the corresponding testosterone response by 180% (testosterone incremental areas 111 ±61 vs 311±49 μg · 1−1 · min−1, p <0.01). These results imply that food deprivation affects the pituitary-testicular axis differently in obese and non-obese men.