Gonadectomy is the commonest surgical procedure carried out in the cat, mainly for fertility control. However, the effect of gonadectomy on body weight in the cat has received little study although this subject is well researched in other species, such as the rat. Part I of the present study involved gonadectomy at 20 weeks. The cats were housed in outdoor colony cages and given food ad libidum, adjusted to leave daily residues. Body weight was measured weekly and transformed to log10. For statistical analysis, differences in body weight of each cat were tested by one-way analysis of variance and serial covariance using the previous week's body weight as the covariate. Differences between groups were investigated with 't' tests and growth rates were studied by regression. Up to 32 weeks of age there was no statistically significant difference between the growth rates of entire versus the castrates in either sex. However, when extended to 55 weeks of age prepubertal gonadectomy in the female cat caused significantly increased growth. This was not observed for the male cat. Little information is available on the anatomy of the brain of the Australasian possum (Trichosurus vulpecula). Part II of the present study aims at presenting a simple description of the possum hypothalamus viewed in three planes of section and concentrating on some of the fibre tracts which are clearly visible. The main findings were that the mammillothalamic tract appears in a similar position to that as seen in other mammals such as the rat, cat, and sheep, while the fornix appears much steeper in its descent into the anterior hypothalumus. In addition, there is described a fibre tract emanating from the optic chiasma and passing to the caudal part of the paraventricular nucleus. This tract has not been described in other mammals, such as the rat, cat, and sheep.