Aflatoxin M1 (AFM1) is a hydroxylated metabolite of aflatoxin B1 (AFB1) and can be excreted in milk of cows after consuming aflatoxin (AF)-contaminated feed. The aim of this research was to assess the levels of total AFs in samples of feedingstuff for dairy cows (n = 193) and the levels of AFM1 in raw bulk tank milk samples (n = 375), in order to estimate the ratio of BAFB1 feed input^ versus BAFM1 milk output^ in four specific regions of Spain. Moreover, the correlation between the raw materials used as ingredients of the total mixed ration (TMR) and the presence of AFs was studied. About one-third (34.7%) of the feed samples were positive for total AFs in a range of 0.05-6.45 μg/kg, and 12.4% were positive for AFB1. AFM1 was detected in 18.9% of bulk milk samples, with concentrations ranging from 0.009 to 1.36 μg/kg. While none of the feed samples exceeded the European Union (EU) maximumcontent for AFB1 in feedingstuff for dairy animals of 5 μg/kg, three bulk milk samples exceeded the EU maximum level for AFM1 in milk of 50 ng/kg. The transfer ratio AFB1/AFM1, which was derived from AFB1 levels in feed, AFM1 levels in bulk tank milk, feed intake, and milk yield data, was 0.6-6%, which corresponded well with the range of published carry-over data for aflatoxins. Statistical analyses showed that the main sources of AFB1 in TMR were maize silage, bagasse, soya bean husk, maize, alfalfa hay, cotton seed and compound feed, thus special attention should be paid in controlling these raw materials when used in TMR preparation. Although the analysis of AFs in feed did not correlate with the presence of AFM1 in milk, monitoring feedstuffs is a useful tool in order to try and minimise AF-contamination of milk.