AbstractA plastic skull with an artificial falx was constructed.A 3 cm square in the temporal area is described which lies perpendicular to most of the 3rd ventricle and pineal body as well as the lower part of the septum pellucidum.A-scan echoencephalograms made from the corners of this square showed that strong echoes either from the falx or the base of the skull were obtained at most angles of incidence of the transducer in the horizontal and coronal planes.A rectangle is described from which it is easier, in a plastic model, to avoid eliciting echoes from the falx and skull base when the transducer is perpendicular to the sagittal plane.An increase in frequency from 1·5 to 2·5 Mc/second accompanied by an increase in receiver sensitivity decreased the ability to scan without eliciting falx or basal echoes.Some of the features of the basal echoes are described. The possibility occurs that they might be elicited during scans of the cerebral mid-line structures and confused with echoes coming from these structures.