The spread of potentials along electrically-coupled horizontal cell networks in the turtle retina was studied at different levels of illumination. The mean values of the stationary space constant, lambda st, measured by the steady-state responses to a bar of light moved along the retina, is about 0.13 mm for the LII horizontal cell response and 0.35 mm for the LI response. lambda st is quite stable in the physiological range of membrane potentials. The dynamic space constant, lambda (t), measured at different intervals after the onset of the light bar, transiently increased during the rising phase of the on-response. Such a transient change of lambda (t) is absent or less apparent for depolarizing responses to the offset of the light bar. Ba2+ ions strongly enhanced the transient increase of lambda (t) for hyperpolarizing responses without affecting lambda (t) for depolarizing responses. Possible mechanisms of the transient increase of the dynamic space constant are discussed.