The chromatic physiology of the stoneloach (Noemacheilus barbatulus L.) was investigated by recording the melanophore index and the transmembrane potential of melanophores using split fin and chromatically intact spinal preparations. Responses of fish to black and white backgrounds, background reversal and darkness show that the transient colour response of this fish is similar to other freshwater teleosts. Adrenergic neurons produce aggregation. Cholinergic neurons appear to play no part in granule movement, although atropine produces dispersion in split fin preparations. Potassium ions produce aggregation which is blocked by yohimbine suggesting that this aggregation is mediated through adrenergic action. Calcium ions are important for both aggregation and dispersion, facilitating the release of aggregating transmitter and acting directly on the cell. The melanophore centrosphere depolarises in aggregating cells and hyperpolarises in dispersing ones. Changes in potential precede granule movement which is slower in dispersing melanophores. Measurement of the melanophore potential at the cell centrosphere and the edge of the pigment show that the cell processes are electrogenically polarised. Also, the sum of these potentials remains nearly constant although the potentials themselves vary with the position of the pigment. The transmembrane potential is related to the transmembrane potassium gradient, although it is also influenced by sodium and calcium ions. The potassium gradient varies locally depending on the reactions involved in maintaining the current pigment position resulting in electrogenic polarisation of the raelanophore. Energy is required for the maintenance of dispersion and membrane potential since ouabain produces aggregation and depolarisation. Low temperature inhibits aggregation and dispersion, although it has little effect on membrane potential. High temperature causes depolarisation probably by cell damage. Microtubules appear to play no part in the maintenance of the membrane potential. However, colchicine produces dispersion of the granules which suggests that inhibition of microtubules by colchicine is incomplete and/or other cytoplasmic elements are able to produce granule movement, or that colchicine has other effects on the cell. It is concluded that changes in the membrane potential and electrogenic polarisation of the cell give significant indications of the nervous and cellular mechanisms involved in the chromatic physiology of teleosts.