Photodynamic therapy (PDT) is an experimental modality in the treatment of cancer. It involves photochemical reactions that require the interaction of a photosensitising drug, light and oxygen. The development of an efficient protocol based on assuring oxygen availability through modulation of the incident light power density and its mode of delivery was addressed in this study. An estimated energy dose of 180 J/cm2 of 630 nm light from pulsed Nd:YAG dye laser was delivered 24 h after injection of 10 mg/kg haematoporphyrin oligomers in C3H/HeNCrj mice bearing the transplantable squamous cell carcinoma NR-S1, by either of these light regimens: (1) 5 mJ/cm2/pulse for 30 min, 1 h dark interval, followed by another 30 min exposure to the same power (low power, periodic light regimen) or (2) 15 mJ/cm2/pulse for 20 min (high power, continuous light regimen). Results showed a higher mean percentage area of tumour destruction with the low power, periodic light regimen at 54.34% in contrast to 12.44% of the high power, continuous light regimen 2 days after PDT. Furthermore, the mean bromodeoxyuridine labelling indices of the remaining viable-appearing cancer cells were 27.90 and 42.41, respectively, indicating a smaller tumour growth fraction with the former regimen. These results suggest that use of low power, periodically delivered light increases the antitumour efficacy of PDT.