Castrate male sheep (wethers, average liveweight 38 +/- 0.6 kg) were given one of the following diets for 10 weeks followed by euthanasia (n = 8/group): high-energy high-protein providing 1-5 times the energy required to maintain liveweight (maintenance) (group 1.5 M), low-energy low-protein at 0.5 maintenance (0.5 M), or low-energy high-protein at 0.5 maintenance (0.5 M + P). 1.5 M wethers gained 22% liveweight whereas 0.5 M and 0.5 M + P wethers lost 18 and 13% liveweight respectively. Relative to the 1.5 M group, the 0.5 M and 0.5 M + P groups had similar plasma concentrations of glucose and cortisol throughout, but elevated non-esterified fatty acids (P < 0.001) and reduced IGF-I and insulin (P < 0.05, 0.01 or 0.001) from 1 week onwards. Each week blood samples were taken every 12 min for 4 h and plasma assayed for LH. Mean concentration over 4 h, LH pulse frequency and LH pulse amplitude showed no progressive change in 1.5 M sheep. However, in both 0.5 M and 0.5 M + P groups mean LH increased (P < 0.001 and P < 0.01 respectively), pulse frequency decreased (P < 0.01 and P < 0.01) and pulse amplitude increased (P < 0.001 and P < 0.01) over the 10-week period. Anterior pituitary. LH content was greater in 0.5 M (P < 0.01) and 0.5 M + P (P < 0.05) than in 1.5 M sheep. Coronal sections (20 microns) of hypothalamic brain tissue were subjected to in situ hybridisation to determine gene expression for neuropeptide Y (NPY). NPY mRNA was concentrated in the arcuate nucleus and median eminence, with total amounts greater in both 0.5 M (310%, P < 0.001) and 0.5 M + P (333%, P < 0.01) groups than in 1.5 M sheep (100%). These data reveal that chronic low dietary energy intake by long-term castrates, with high or low protein intake, reduces LH pulse frequency but increases the circulating levels of LH by virtue of an increase in pulse amplitude, and concomitantly increases hypothalamic NPY gene expression.