Precise hypothalamic surgery in the marmoset monkey Callithrix jacchus cannot be performed by means of the Horsley-Clarke stereotaxic procedure. Thus, a simple stereotaxic X-ray technique was developed. Technical requirements are a stereotaxic frame for small animals and a dental X-ray source. A sagittal map shows hypothalamic structures in relation to surrounding bony landmarks. Corresponding coronal plates allow target determination in that plane. Intracerebral coordinates based on the position of the optic chiasma can be transformed into individual Horsley-Clarke coordinates. The error of the atlas is estimated as +/- 0.25 mm anteroposteriorly, +/- 0.1 mm coronally, and +/- 0.5 mm vertically.