In order to define neuronal damage produced by sham, frontal and frontolateral hypothalamic knife cuts known to effect neuroendocrine and behavioral variables, we designed a retractable knife capable of delivering horseradish peroxidase as a cut was being produced. Damaged neurons filled with homogeneous reaction product were found in virtually all hypothalamic nuclei. A similar distribution of damage was produced by frontolateral (FLC), frontal (FC) or sham cuts--the primary differences being the greater density of damaged neurons in hypothalamic nuclei and labeling of the ventral tegmental area of Tsai in FLC and FC animals. The difference between FC and FLC animals was primarily one of degree of damage. The amount of damage produced by the sham surgery is consistent with the idea that sham surgeries produce endocrine dysfunctions intermediate between frontal cut animals and unoperated controls. It is interesting to note that a number of nuclei damaged by these surgeries provide aminergic innervation to the hypothalamus.