The impact of lipophilicity on biological parameters critical to photodynamic efficacy was analyzed for a new generation of trisulfobenzo(mononaphtho)porphyrazines. The porphyrazines were substituted on the naphtho ring with linear alkynyl side chains of various lengths. When compared to the analogous phthalocyanine structures, the added benzo ring in the porphyrazine structures increased the lipophilicity for analogs with short alkynyl chains, while this effect disappeared for analogs with longer side chains. In aqueous media, the analogous phthalocyanine series showed aggregation tendencies. In contrast, no correlation between aggregate formation and the length of the alkynyl side chain was evident in the porphyrazine series. At low concentrations, the length of the side chain did not affect cell uptake, while phototoxicity towards EMT-6 mouse tumour cells showed a parabolic relationship, where the hexynyl derivative showed the highest activity. The trisulfonated porphyrazines localized at intracellular organelles, plasma and perinuclear membranes, but could not be found in the nucleus. Total cell uptake of dye did not correlate with phototoxicity, suggesting that localization in certain intracellular organelles, and distribution into critical intracellular sites are important determinants of their photodynamic activity. The hexynyl trisulfonated zinc porphyrazine derivative (ZnNPcS₃C₆) showed the strongest in vitro photodynamic activity and therefore was further studied in an EMT-6 mouse tumour model. An i.v. dose of 1 μmole of ZnNPS₃C₆ per kg, followed 24 h later by activation with light, induced 100% tumour necrosis within 24 h post-PDT. This treatment delayed tumour volume doubling time from 5 days to >2 weeks, and gave 41% tumour cure >3 weeks post-PDT. Applying the same light dose fractionated (5 min on, 2 min off), further improved tumour response, leading to a doubling time of 26 days and a 73% tumour cure. At the administered 1 μmol kg⁻¹ dye dose, no skin phototoxicity was observed and >90% blood clearance was observed within 5 h post-injection. Compared to the analogous trisulfo monohexynyl zinc phthalocyanine, the new trisulfobenzo(mononaphthohexynyl)porphyrazine provided a broader range of excitation wavelengths, and improved photodynamic potency, while apparently being free of unwanted systemic side effects.