OBJECTIVE Our purpose was to measure any short-term effects that the transdermal nicotine replacement system may have in pregnancy and to verify salivary nicotine and cotinine levels during patch placement. METHODS After customary smoking cessation efforts had failed, six prenatal patients between 28 and 37 weeks' gestation who smoked between one and two packs per day were enrolled in this prospective study. The patients were admitted to the General Clinical Research Center for a period of 21 hours. During hospitalization we performed maternal and fetal assessments including vital signs, biophysical profile and electronic fetal monitoring, amniotic fluid index, and umbilical artery Doppler examinations. Salivary samples for cotinine and nicotine levels were collected at standard intervals. RESULTS There were no measurable differences in fetal or maternal well-being. During patch use salivary nicotine levels increased as expected, to a mean value of 19.0 +/- 13.5 micrograms/L at 480 minutes. Salivary cotinine concentrations remained low (approximately 50 micrograms/L) and varied little during the 480-minute period that the patch was worn. Overall, patients were satisfied with the transdermal patches. CONCLUSIONS There were no adverse maternal or fetal effects from the transdermal nicotine replacement system over the 6-hour period. Salivary nicotine concentrations were consistent with those seen in nonpregnant adults. Surprisingly, salivary cotinine concentrations were much lower than those seen in smoking nonpregnant adults.