A system for exposing rats to mainstream (MS) and sidestream (SS) smoke simultaneously from the same cigarette, and monitoring procedures, are described in detail. The equipment and procedures were used to expose Sprague-Dawley rats to mainstream smoke and to target deliveries of 10, 25, or 50% of the total SS smoke for 17 weeks. The estimated total particulate matter (TPM) dose was highly correlated with the increase in percent COHb for MS and SS smoke, but the COHb/TPM relationships were different for the 2 kinds of smoke. All SS TPM doses were much lower than the MS TPM dose, and the COHb/TPM ratio for SS smoke was much higher than for MS smoke. The TPM dose and per cent COHb for SS smoke were highly correlated with the per cent of SS sent to the exposure chambers. There were no significant differences in the total weight changes during the study for any of the smoke exposed groups, but weight changes during the 12-17-week period for sidestream exposed groups were inversely correlated with the level of sidestream exposure.