N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was previously used to study the kinetics of the OJIP chlorophyll fluorescence rise. The present study is an attempt to elucidate the origin of TMPD-induced delay and quenching of the I-P step of fluorescence rise. For this purpose, we analyzed the kinetics of OJIP rise in thylakoid membranes in which electron transport was modified using ascorbate, methyl viologen (MV), and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). In the absence of TMPD, the OJIP kinetics of fluorescence induction (FI) was not altered by ascorbate. However, ascorbate eliminated the I-P rise delay caused by high concentrations of TMPD. On the other hand, neither ascorbate nor DBMIB, which blocks the electron release from Photosystem II (PS II) at the cytochrome b6/f complex, could prevent the quenching of I-P rise by TMPD. In control thylakoids, MV suppressed the I-P rise of FI by about 60. This latter effect was completely removed if the electron donation to MV was blocked by DBMIB unless TMPD was present. When TMPD intercepted the linear electron flow from PS II, re-oxidation of TMPD by photosystem I (PS I) and reduction of MV fully abolished the I-P rise. The above is in agreement with the fact that TMPD can act as an electron acceptor for PS II. With MV, the active light-driven uptake of O2 during re-oxidation of TMPD by PS I contributes towards an early decline in the I-P step of the OJIP fluorescence rise.