The fluorescence lifeime of a number of porphyrin free bases and metal-loporphyrins has been measured on a single photon counting apparatus.The HPD bulfer solution used as the sensitizer for the photodynamic therapy of cancer displayed a double exponential decay
with a short lifetime component of τ 
3ns characteristic of aggregated HPD and a long liietime component of τ 14ns characteristic of the monomeric HPD;at the mean while
the HP buffer solution displayed a single exponential decay.The lifetimes of seven porphyrins or their metal complexes were measured for the first time.The measured lifetime of several substances are in good agreement with the calculated values.