Tau is a neuronal microtubule-associated protein. Its hyperphosphorylation plays a critical role in Alzheimer disease (AD). Expression and phosphorylation of tau are regulated developmentally, but its dynamic regulation and the responsible kinases or phosphatases remain elusive. Here, we studied the developmental regulation of tau in rats during development from embryonic day 15 through the age of 24 months. We found that tau expression increased sharply during the embryonic stage and then became relatively stable, whereas tau phosphorylation was much higher in developing brain than in mature brain. However, the extent of tau phosphorylation at seven of the 14 sites studied was much less in developing brain than in AD brain. Tau phosphorylation during development matched the period of active neurite outgrowth in general. Tau phosphorylation at various sites had different topographic distributions. Several tau kinases appeared to regulate tau phosphorylation collectively at overlapping sites, and the decrease of overall tau phosphorylation in adult brain might be due to the higher levels of tau phosphatases in mature brain. These studies provide new insight into the developmental regulation of site-specific tau phosphorylation and identify the likely sites required for the abnormal hyperphosphorylation of tau in AD.