The pattern-related capacity for the dispersion of previously aggregated melanosomes in low concentrations (3 × 10−6 to 10−8 M) of noradrenaline in vitro was observed in melanophores from winter flounder Pseudopleuronectes americanus. With 10−8 M noradrenaline, dispersion was completed more rapidly than in controls using the incubation vehicle alone. Melanophores from white-spot, dark-band and general background components of the integumentary pattern displayed different ‘transition ranges’ between melanosome aggregation and dispersion in higher and lower concentrations of noradrenaline. Within each ‘transition range’ individual noradrenaline concentration decrements could result in highly variable degrees of melanosome dispersion. The relative breadth of the noradrenaline ‘transition range’ concentrations could be represented as dark bands > general background > white spots. The threshold noradrenaline concentration for dispersion was highest for the dark bands. It is concluded that these differences represent variations in the transition from melanophore α-adrenoceptor-mediated pigment aggregation to -adrenoceptor-mediated dispersion between localized areas of the skin. Such variations in ‘transition range’ will have an important role in the expression of flatfish patterns and in their changes in colour and texture.