Time-resolved confocal microspectrofluorometry and fluorescence microimaging were used to monitor how the model antisense oligonucleotide is transported into 3T3 living cells and distributed inside them. Phosphorothioate analog of 15-mer oligothymidylate labeled by ATTO 425 was complexed with Zn(II) 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin as an uptake-mediating agent. Homodyne phase-resolved technique based on a high frequency analog modulation of both exciting diode laser and detector image intensifier was used for time-resolved measurements. Decay-time data obtained within a broad range spectral region have provided unique information about the fate of both fluorophores inside the cell.