It was shown previously that the spectral sensitivity of luminosity/H1 -type horizontal cells (HCs) in carp retinae reflects the absorption spectrum of red-sensitive cones for long wavelengths but can appear highly variable and‘truncated’in the short-wavelength region of the spectrum. We have found that light-adaptation sharpened the red-sensitive spectral peak and decreased the blue/red response amplitude ratio (B/R ratio), mainly by decreasing the response to short-wavelength stimuli. The adaptation effect was more pronounced for red background light than for blue. During dark adaptation, the B/R ratio increased steadily. Exogenous dopamine (DA; 5 μM) changed the spectral response profile in a similar way to light-adaptation. However, the effect of light-adaptation in reducing the B/R ratio was still seen in retinae bathed in 5 μM DA. This effect of background adaptation was also recorded in retinae bathed in 37 μM haloperidol, as well as in retinae pretreated with 6-hydroxydopamine (i.e. DA-depleted). The results suggest that (i) short-wavelength-sensitive cones play a dynamic role in determining the spectral response profile of H1 HCs and (ii) spectral response characteristics are modulated independently by exogenous DA and an unknown endogenous neuromodulator which is activated by light-adaptation.