In this study a stereotaxic instrument and a stereotaxic procedure based on external skull structures, to be used in prepubertal male Landrace pigs weighing less than 30 kg, is described. The instrument represents an adaptation of the apparatus designed by Marcilloux et al., Brain Res Bull 1989;22:591-597, but we have modified the instrument for stereotaxic procedures based on external skull structures, instead of intracerebral structures necessitating ventriculography (Marcilloux et al., Brain Res Bull 1989;22:591-597). For this reason the U-shaped frame and the ear-bar supports have been changed allowing the three-dimensional placement of the ear-bars into the oblique auditory canals. Firm fixation of the skulls of pigs weighing less than 30 kg, was furthermore secured with modified infraorbital ridges and hard palate pieces. Measurements of distances between external skull structures in animals of the same sex, age and weight showed a negligible variation, thus enabling definition of the horizontal, frontal and sagittal zero planes using external skull structures alone. Stereotaxic coordinates for the hippocampal region of male Landrace pigs weighing 10 kg were then provided and the coordinates from two different levels of the hippocampal region are presented in the text. The reliability of the stereotaxic instrument was finally secured by intrahippocampal injections of ink at predetermined coordinates.