The influence of the time interval between dye administration and detection by fluorescence microscopy was assessed in "early" squamous cell carcinomas of the cheek pouch mucosa and different healthy tissues of the Syrian hamster. Following intracardiac injection of 0.15 mg (kg body wt.)-1 of meso-tetra-hydroxyphenylchlorin (m-THPC), groups of three animals were sacrificed at different time intervals up to 30 days. A group of three non-injected animals was used to detect the endogenous fluorescence of the corresponding normal tissues for autofluorescence subtraction. The following excised organs (esophagus, trachea, liver, spleen, kidney, skin, striated muscle, healthy and tumoral cheek pouch mucosae) were fast frozen in liq. nitrogen and stored at -70 DegC for fluorescence microscopy. The results show significant differences in the detectable m-THPC levels in different tissue layers (for instance, the epithelia and muscle of the esophagus, trachea and cheek pouch) at different time intervals. These data indicate that pharmacokinetic studies may be useful for selecting the optimal time for the photodetection and phototherapy of cancer.