An inhalation bioassay with Syrian golden hamsters is being conducted to evaluate the toxic and carcinogenic potential of cigarette sidestream smoke (SS) relative to mainstream smoke (MS). A Hamburg II smoking machine is used to deliver MS by nose-only exposure to hamsters and a modification allows for the simultaneous collection of SS for whole-body delivery to a different rack of animals. The tolerated dose of SS was determined by varying the air/smoke dilutions drawn through the animal restrainers. Preliminary data indicated that 20% carboxyhemoglobin (COHb) could be obtained in SS-exposed animals without fatality. Optimum exposure levels were determined. Monthly measurements of COHb, nicotine and cotinine indicate that the SS-exposed animals are absorbing slightly higher amounts of these smoke constituents than the MS-exposed hamsters. Tumor incidence and carcinogenicity data are being collected through complete necropsy and histology protocols and uptake data continue to be collected. These studies should help elucidate the carcinogenic potential of SS which has been suggested from its composition and from recent epidemiological data of cancer incidence in non-smokers.