A method has been developed for recording the response of single neurons in the lamprey brainstem in vitro to natural stimulation of vestibular receptors. The brainstem dissected together with the intact vestibular apparatus could be rotated in space, in two perpendicular planes (transverse, the roll tilt, and sagittal, the pitch tilt), in one of them up to 360 degrees, and in the other one up to +/- 30 degrees. The responses of single reticulospinal (RS) neurons, in all four reticular nuclei of the brainstem, to roll and pitch were recorded extracellularly and, with small inclinations (up to +/- 45 degrees) also intracellularly. Two types of preparations were used, with and without the rostral part of the spinal cord. In the brainstem preparations, most RS neurons responded both to a definite brain orientation in space and to a change of the orientation (static and dynamic reactions). Responses to roll tilt were similar in all reticular nuclei: all cells were excited with roll tilt towards the contralateral side, this reaction was qualitatively preserved when the roll was performed in combination with different pitch inclinations. Responses to pitch tilt were less clearcut; some neurons were activated with nose-up deflection while others responded to nose-down tilt. In preparations including the spinal cord, responses of RS neurons to roll and pitch tilt differed from those in the isolated brainstem in that they were much less specific and stable. Roll and pitch tilts could trigger the spinal locomotor CPG, which, by sending "efference copy" signals back to the brainstem, produced modulation of RS neurons in relation to the locomotor rhythm.