The present study was conducted to determine if: 1) preovulatory yolks may be an important storage depot for drug residues in eggs laid days to weeks after drug withdrawal and 2) the model developed by this laboratory (Donoghue et al., 1996) is predictive of the pattern in eggs laid during and after drug withdrawal. Twenty-four hens were dosed for either 1, 2 or 3-d with ampicillin (40 mg/kg, I.M.). Ampicillin was not detectable in plasma 24 h after injection for any group (assay sensitivity of 0.6 ppb). Hens from all three groups produced eggs containing detectable ampicillin residues for 6 d after drug withdrawal. Because plasma ampicillin was not detectable even 24 h after final dosing, the majority, if not all, of the incurred ampicillin residues contained in laid eggs after drug withdrawal were due to incorporation and storage of drug in preovulatory yolks during the dosing period. Additionally, the pattern of residues in laid eggs was consistent with our model. These data emphasize the importance of transfer and storage of drugs in preovulatory yolks as a significant contributing mechanism for the production of incurred drug residues in eggs.