In the visual cortex of cats which had been dark-reared for several months since the time before natural eye opening, a cortical infusion of 6-hydroxydopamine (6-OHDA), a noradrenaline (NA)-related neurotoxin, partially suppressed a usual shift in ocular dominance following brief monocular lid suture, causing a significant loss of binocular cells. This partial shift in ocular dominance (U-shaped histogram) was also observed typically in the control hemisphere of cats which were subjected to dark-rearing for more than a year. Furthermore, the expected shift in ocular dominance following monocular deprivation was blocked by a direct cortical infusion of D,L-metoprolol, a selective beta 1-adrenergic receptor antagonist. The blockade was not obtained by D-metoprolol, a biologically inert stereo-isomer, under the comparable condition. In contrast, exogenous L-NA gave rise to an obvious shift in ocular dominance toward the non-deprived eye. The present results suggest that the NA-beta 1 adrenoreceptor system was necessary to maintain the ocular dominance plasticity in the visual cortex of dark-reared cats.