ABSTRACT The motion of the endolymph in the semicircular canal is described with the aid of a linear second-order system, comprising the applied angular acceleration and the linear displacement, the linear velocity and the linear acceleration of the endolymph. A sensitivity factor G for angular velocity is derived and expressed in the dimensions of the semicircular canal. The sensitivity factor G of the semicircular canal proves to be consistent with the equivalent sensitivity factor of a more sophisticated hydrodynamic model of these canals. The sensitivity of the growing semicircular canal is defined to be , independent of the pike’s size. Threshold angular velocities for the vestibulo-ocular reflex of 27 pike between 4 and 50 cm bodylength are in agreement with this assumption. At threshold stimulation with an angular velocity of 2°/sec the radius of curvature Rc of the cupula is calculated to be 220 ± 48 cm. At the threshold angular velocity γ = 2°/sec the deviation of sensory hairs 5 μ m long is assessed to be between 0·06 Å and 1 Å. Young’s modulus of elasticity for the cupular substance is found to be between 0·35 × 103 dyne/cm2 and 1·85 × 108 dyne/cm2 (on the basis of a circular bending for the cupula). Characteristics of the model of the growing semicircular canal are calculated for pike with body length between 5 and 100 cm. Observed growth effects of the pike’s vestibulo-ocular reflex arc are correlated to the growth effects calculated on basis of the model for the growing semicircular canal. The pike possesses the same degree of vestibulo-ocular compensation in the high-frequency range (2 radians/sec = ⩽ ω< 7 radians/sec) of angular oscillations during its whole life (11 observations of pike of body length 4–56 cm).