Photodynamic therapy (PDT) utilizing hematoporphyrin derivative (Hpd) as photosensitizer and an argon-dye laser as the light source was used alone and in combination with a localized microwave hyperthermia treatment to treat the SMT-F mammary carcinoma in mice. A 30-min heat treatment at 44.5 degrees C was applied 0-8 hr before or after a standard photodynamic treatment (67.5 or 135 J/cm2, given 24 hr post-7.5 mg/kg Hpd). Potentiation of PDT by heat was found to be related to the sequence of the treatments and the time interval between them. When 44.5 degrees C for 30 min was applied immediately after a 15-min PDT treatment, significant potentiation was seen (58% long-term tumor control vs 3 and 10%, respectively, for PDT and heat alone). This potentiation decreased with increasing time between PDT and heat, with tumor control values decreasing to 36, 20, and 14%, when 2, 4, and 8 hr, respectively, were allowed between treatments. Only additive effects of the independent therapies were found when this heat treatment was applied 0-8 hr before PDT. In other experiments, mice were treated with single or fractionated 30-min PDT treatments (two 15-min treatments separated by 0-, 2-, 4- or 8-hr intervals). Decreases in tumor control were seen with increasing time interval; only minor differences in tumor control were seen when 4-8 hr was allowed between treatments compared to a single 15-min treatment.