The objectives of the present study were (1) to monitor the pattern of excretion of aflatoxinM1 in urine after its conversion from aflatoxinB1 and (2) to observe the effects of different levels of aflatoxinB1 in feed on serum concentrations of key metabolites glucose, total protein, cholesterol and urea as indicators of metabolic status. Nili-Ravi buffalo heifers (n = 12) of similar age and weight were randomly distributed to four groups. Animals in Groups A, B and C were offered a contaminated cottonseed cake-based concentrate ration at 0.5%, 1.0% and 1.5% of bodyweight, respectively. Control animals in Group D were fed with aflatoxinB1-free green fodder. Based on the level of contamination of the concentrate ration with aflatoxinB1 (554 mg/kg), Groups A, B and C consumed 953, 2022, 3202 mg of aflatoxinB1 daily. Feed samples were analysed at Romer Laboratories Pty Ltd, Singapore by high performance liquid chromatography. AflatoxinM1 quantification in urine samples was conducted using a competitive enzyme-linked immunosorbent assay with kits supplied by Helica Biosystems, Inc., USA. Serum samples were analysed for concentrations of glucose, total protein, cholesterol and urea using clinical chemistry kits provided by Human diagnostics (HUMAN, Biochemica und Diagnostica mbH, Germany). Carry-over rate of aflatoxinM1 in urine for Groups A, B and C was 15.51%, 15.44% and 14.04% of aflatoxinB1 while there was no detectable aflatoxinM1 in the urine of the control group (D). There was no significant difference in the concentrations of serum glucose, total protein and cholesterol between treatment groups. However, the concentration of serum urea was significantly higher (P < 0.05) in the group offered the highest level of aflatoxinB1- contaminated concentrate. This result suggests that mycotoxicosis may compromise protein metabolism and accretion in affected animals. This leaves open the possibility that high concentrations of aflatoxins in milk may ultimately affect the health status of human milk consumers.