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 <img style="vertical-align:middle" alt="" order="0" src="http://www.rhhz.net/qikantupian/fgxb-19880111-74.gif"/> 3ns characteristic of aggregated HPD and a long liietime component of τ <img style="vertical-align:middle" alt="" order="0" src="http://www.rhhz.net/qikantupian/fgxb-19880111-74.gif"/> 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.