The effectiveness of low-fluence rate PDT on human glioma spheroids incubated in 5-aminolevulinic acid (ALA) or hexyl- ALA, is investigated. Spheroid survival is monitored following PDT at representative drug concentrations, light fluences and fluence rates. It is shown that, for above threshold fluences, significant PDT response can be obtained at a fluence rate of 5 mW cm-2. At this fluence rate, the response of human glioma spheroids to PDT with lipophilic ester derivatives, such as hexyl-ALA, is equivalent to that obtained with ALA, however, this equivalency is observed for ester concentrations 20 times lower than the parent compound. The efficacy of PDT is significantly reduced at a sub-threshold fluence of 25 J cm-2. Spheroid response appears to be somewhat fluence rate dependent at sub-threshold fluences - survival is marginally poorer at the higher fluence rate (25 mW cm-2) investigated. In addition, the efficacy of PDT at 25 J cm-2 depends significantly on photosensitizer concentration at the lowest fluence rate investigated (5 mW cm-2).