The steady-state density and the turnover rates of D1-dopamine receptors were investigated in the striatum, nucleus accumbens, substantia nigra, and retina of adult (3-month-old) and aged (23-month-old) rats. The turnover rates were measured by monitoring the repopulation kinetics of D1-dopamine receptors labeled with [3H]-SCH 23390 after the irreversible inactivation induced by a single dose of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 10 mg/kg, s.c.). In all the neural tissues examined, the repopulation of D1 dopamine receptors could be adequately described by a theoretical model that assumes a constant rate of receptor production (i.e. zero order) and a rate of degradation that is dependent on the receptor density at any time (i.e. first order). The results obtained indicate that the reduction in the density of D1-dopamine receptors in the striatum, nucleus accumbens and substantia nigra of aged rats is the result of a larger decrease in the receptor production rate (-44 to -60%) than in the receptor degradation rate (-21 to -46%). By contrast, the production rate of D1-dopamine receptors in the retina of aged rats remains unchanged, whilst the degradation rate is reduced by 25%. This results in an age-related increase in the density of D1-dopamine receptors in the rat retina.