Earlier we reported defects in D1 receptor function in renal proximal tubules (RPTs) of aged Fischer 344 (F344) and obese Zucker rats. However, the defects in the receptor function in RPTs of obese Zucker rats do not pass onto primary cultures of RPTs from these animals. Here, we determined whether the defects in D1 receptor function in RPTs of aged F344 rats pass onto the primary cultures. RPTs from aged (24-month) and adult (6-month) F344 rats were grown into primary cultures. The microscopic studies showed that cells in cultures from adult and old rats were healthy as determined by the shape and size of the cells and nuclei. D1 receptor agonist SKF 38393 produced inhibition of ⁸⁶Rb (rubidium)-uptake, index of Na,K-ATPase activity, in cells from adult rats but this was reduced in old rats. Also, SKF 38393 increased the ³⁵S-GTPS binding, index of receptor activation, in the membranes of cells from adult rats but to a lesser extent from old rats. Furthermore, there was a downward trend in the levels of D1 receptor numbers and the receptor proteins in old rats. Interestingly, gp^91phox subunit of NADPH oxidase and cellular protein carbonyl levels (oxidative stress marker) were higher in cultures from old rats. These results show that RPTs from adult and old F344 rats grow into epithelial cells in cultures. Furthermore, cells in cultures from old rats are at a higher level of oxidative stress, which may be contributing to the reduced D1 receptor function in the cells from old compared to adult rats.