In previous studies, we have observed unexpected structure-tumorigenicity relationships among the dimethylchrysenes. Thus, 5,6-dimethylchrysene and 5,7-dimethylchrysene were only weakly tumorigenic and were significantly less active than 5-methylchrysene. These results were surprising in view of the known route of metabolic activation of 5-methylchrysene via its 1,2-diol 3,4-epoxide. In this paper, we extended our studies of structure-tumorigenicity relationships among the dimethylchrysenes. We synthesized 5,7-, 5,8-, 5,9-, and 5,10-dimethylchrysene via photochemical ring closure reactions. The tumor-initiating activities of these dimethylchrysenes on mouse skin were compared with those of 5-methylchrysene and 5,6-dimethylchrysene. 5- Methylchrysene and 5,9-dimethylchrysene were highly tumorigenic and were significantly more active than 5,6-, 5,7-, 5,8-, and 5,10-dimethylchrysene. The results of these studies, taken together with those reported in the subsequent two papers, suggest that the molecular shapes of dimethylchrysenes influence the balance between metabolic activation and detoxification pathways.