A new animal model has been developed using the Holstein cow and her own calf to monitor offspring exposures to drugs in utero and/or through consumption of mother's milk. Two drugs used in human medicine, phenylbutazone and ivermectin, were tested as prototype drugs in the cow-calf model. Both drugs have long elimination half-lives in the human and the bovine, and both are secreted into milk at low levels, such that the resulting estimated dose to the nursing human infant is 5% of the maternal dose per day for phenylbutazone and 2% of the maternal dose per day for ivermectin. The literature suggests that the use of these drugs is compatible with breast-feeding. In the cow-calf model, lactational exposure to 1% of the maternal dose of phenylbutazone twice daily resulted in appreciable accumulation of the drug in the calf's plasma after a 7-day exposure period. A lactational exposure to 3% of the maternal dose twice daily, for 7-days, maintained a steady-state plasma level in a calf that was also exposed to phenylbutazone in utero. For ivermectin, a lactational exposure to 0.5% of the maternal dose of ivermectin twice daily resulted in plasma drug accumulation in calves approximating 50% of the adult therapeutic blood level after 7-days exposure. Thus the cow-calf model dramatically demonstrates that exposures of newborn to low levels of phenylbutazone or ivermectin, or to drugs with similar physico chemical characteristics, in breast milk may not be considered safe due to the potential for these compounds to accumulate in the newborn.