Cotinine is the gold standard biomarker for exposure to tobacco smoke.1 As the primary metabolite of nicotine and with a biological half-life of 20 hours, testing plasma and saliva for cotinine concentration has become commonplace. Cotinine assays for dried blood spots (DBS) have recently been generating interest as well, especially for measuring newborn dried blood spots to determine fetal exposure to maternal smoking.2 DBS show promise as a cost-effective method of sample collection in a variety of population-based studies of environmental exposure. Here, we demonstrate methods for measuring cotinine in plasma and DBS samples and use them to determine exposure to tobacco smoke among pregnant women in one such population-based environmental study. Cotinine levels in 153 plasma samples were analyzed using LC-MS/MS. This method was then adapted to measure cotinine in DBS using a Pressure BioSciences Barozyme HT48 to expedite sample preparation and a nanochip LC interface for greater LC-MS sensitivity.