Adaptability of the horizontal vestibulo-ocular reflex (HVOR) and the optokinetic response (OKR) was examined in alert albino rabbits during sustained runs lasting 5--12 h under four different stimulus conditions. (1) Sinusoidal rotation of the rabbit in darkness by 5 degrees at 1/10 Hz, or (2) sinusoidal movement of a vertical slit light by 2.5 degrees of 5 degrees at 1/10 Hz around the optical axis of the stationary rabbit, affected the gain of neither the HVOR nor the OKR. (3) Combination of the stimulus as in (1) with the stationary slit light increased the gain of the HVOR gradually. A plateau at about 140% of the initial control was reached in 5 h. (4) Combination of the stimulus as in (1) with the slit light movement by 10 degrees in phase with the turntable decreased the HVOR gain gradually, a plateau being obtained at about 70% of the initial control in 5 h. Changes of the HVOR gain induced in conditions (3) and (4) were not frequency-specific and accompanied by no signficant modification of either the gain or phase of the OKR or the linear property of HVOR-OKR interaction. A small but significant change of the HVOR phase was also detected under the condition (3) but not (4).