tau protein kinase I (TPKI) phosphorylates tau and forms paired helical filament epitopes in vitro. We studied temporal expression and histochemical distribution of tau phosphoserine epitopes at sites known to be phosphorylated by TPKI. Antibodies directed against phosphorylated Ser199 (anti-PS 199) or phosphorylated Ser396 (C5 or anti-PS 396) were used. TPKI is abundantly expressed in the young rat brain and the highly phosphorylated juvenile form of tau occurs in the same period. The activity peak of TPKI coincided with the high level of phosphorylation of Ser199 and Ser396 in juvenile tau at around postnatal day 8. By immunohistochemistry on the hippocampus and neocortex of 3-11-day-old rats, phosphorylated Ser396 was found in young axonal tracts and neuropil, where TPKI immunoreactivity was also detected. TPKI and phospho-Ser199 immunoreactivities were also detected in the perikarya of pyramidal neurons. TPKI immunoreactivity had declined to a low level and phosphorylated serine immunoreactivities were undetectable in the sections of adult brain. These findings implicate TPKI in paired helical filament-like phosphorylation of juvenile form of tau in the developing brain.