The activity of the endogenous retinal dopamine (DA) pathway has been examined in the pigmented rat using retinas obtained from normal adult (∼3 months) and senile adults (∼24 months) using an in vitro electrophysiological approach. By comparing the pharmacological sensitivity of the horizontal cells (HCs) to exogenous DA, a D1 receptor antagonist (SCH 23390) and a DA-transport inhibitor (nomifensine), it is suggested that there is a functional deficit in the endogenous DA activity in the senile retina. Cells recorded from retinae obtained from senile animals are more sensitive to exogenous DA, whilst the senile retina is insensitive to SCH 23390. In addition, nomifensine was effective in potentiating subthreshold DA applications, but only in the normal adult retina. The data may suggest that endogenous DA release upon the HCs and selective re-uptake are suppressed in these retinae. These functional deficits also appear to be associated with changes in the receptive fields of the HCs, suggesting there is a corresponding deficit in spatial processing at the outer plexiform layer (OPL) of the senile rat.