The chick chorioallantoic membrane (CAM) model was used to study synergistic effects of photodynamic therapy (PDT) and hyperthermia (HPT). Since HPT is known, and PDT is believed, to involve a vascular mechanism, the CAM is an ideal medium to study the synergism of these modalities. Moreover, the CAM is a particularly convenient model to manipulate the PDT and HPT parameters and to monitor the modifications of the vasculature: (1) It is possible to view individual blood vessels in the CAM and to examine structural changes in real time. (2) The CAM is a closed system in which HPT can be performed quantitatively and to a selected depth, using different lasers. And (3) variations of surface temperature during PDT + HPT can be readily monitored by noninvasive radiometric techniques. A porphyrin-type photosensitizer solution was applied to areas of the CAM, defined by teflon O-rings placed on the surface. Uptake of the sensitizer into the CAM was determined by monitoring its fluorescence. Excitation light at 405 nm from a spectrofluorometer was directed onto the CAM surface using a bifurcated fiberoptic light guide which also transmitted the fluorescence from the CAM area. The fluorescence-emission spectrum (630-730 nm) and intensity at different times following sensitizer application was measured in vivo. This technique permitted the determination of the uptake dynamics of the sensitizer in the CAM and the establishment of the optimal time for irradiation. After an equilibrium time of 30 minutes, to allow for uptake of sensitizer in the CAM, the area was irradiated with a dual-wavelength system composed of a dye laser at 644 nm (to induce PDT) and a CO2 laser at 10.6 micrometers (to bring about HPT). Damage to the CAM vasculature, due to combined PDT + HPT, was compared to the outcome of the separate modalities. The observed synergistic effect of about 30 was interpreted by invoking various physiological processes. The egg, being a closed in vivo system, lends itself to mathematical modeling of the temporal and spatial temperature profile. The importance of heat dissipation due to diffusion, radiation, and blood perfusion was shown to be small compared to that of heat dissipation due to evaporation of water from the CAM.