Most xenobiotics are transferred into milk by passive diffusion; however, some drugs have been reported to accumulate in milk as a result of active transport. In the present study, lactating Sprague Dawley rats were used to characterize the transfer of nitrofurantoin into milk. The observed milk to serum concentration ratio (M/S) of 31.1+/-4.0 was 100 times higher than the M/S predicted by diffusion (0.3+/-0.1), indicative of an active transfer into milk. Randomized crossover infusions of nitrofurantoin (0.5mg/h) in the absence and presence of a cimetidine infusion regimen (15mg/h) resulted in the corresponding mean M/S of 29.5+/-5.4 vs. 30.7+/-5.0 and systemic clearance (Cls) of 2.7+/-0.7 vs. 2.2+/-0.4 L/h/kg, respectively. Nitrofurantoin infusions (0.5mg/h) in the absence and presence of a higher cimetidine infusion regimen (30mg/h) resulted in the corresponding mean values for M/S of 23.0+/-7.7 vs. 19.8+/-5.9 and Cls of 2.8+/-0.4 vs. 1.4+/-0.4L/h/kg, respectively. Only the decrease in Cls at the higher cimetidine infusion was statistically significant. These observations provide evidence that nitrofurantoin is actively transported into rat milk by a transporter that is not inhibited by cimetidine. These data suggest the presence of at least two distinct mammary epithelial transporter systems, one that transports organic cations (e.g., cimetidine) and another for anions (e.g., nitrofurantoin).