Neuropeptide Y (NPY)-producing neurons in the hypothalamic arcuate nucleus (ARC) have been implicated in growth hormone (GH) feedback in several studies in rats. Ames (df/df) and Snell (dw/dw) dwarf mice carry mutations in transcription factors Prop-1 and Pit-1, respectively, that abrogate detectable expression of GH, prolactin (PRL) and thyroid stimulating hormone (TSH). The present study was undertaken to determine whether and to what extent hypothalamic NPY neurons are affected by the lifelong absence of pituitary hormone feedback in hypopituitary Ames and Snell dwarf mice. Total ARC NPY mRNA levels were quantified using in situ hybridization (ISH) and numbers of ARC NPY-producing cells were quantified using immunocytochemistry (ICC). For ISH, dwarf and normal coronal brain sections were hybridized with ³⁵S-labeled riboprobe complementary to rat NPY cDNA, then analyzed for total signal intensity from the entire ARC for each animal as well as for mRNA per neuron. NPY-containing perikarya in ARC were counted in sections of colchicine-treated (intracerebroventricular) dwarf and normal mice. Total ARC NPY mRNA was reduced in df/df mice to 33.6% (P < 0.01) of that in normal littermates, and reduced in dw/dw mice to 46.3% (P < 0.05) of normals, but there was no difference in expression per neuron as determined by reduced silver grain counting. The decrement in dwarf mice of total ARC NPY mRNA without reduction in mRNA per cell suggested a reduction in NPY-containing neuron number, which was the case as shown by ICC. NPY neuronal number in adult Ames dwarf mice (1048 ± 104) was significantly (P < 0.01) reduced to 68% that in DF/df littermates (1536 ± 73), and significantly (P < 0.05) reduced in Snell dwarf mice to 63% of normals (1138 ± 137 vs. 1726 ± 205). This study represents the first enumeration of NPY-producing neurons in mouse hypothalamus and the first demonstration of lower NPY neuron number in a hypopituitary model. The reduction in total NPY mRNA was greater than that reported in studies of GH-deficient rats, suggesting that NPY expression may be affected by the lifelong absence of PRL or TSH or both, as well as GH.