Eighteen beef heifers, representing the Bos taurus (Shorthorn), Bos indicus (Afrikaner) and intermediate (Bonsmara) types, were confined to a pen with 4,6 m² floor space per heifer. A similar number of heifers on veld grazing served as controls. Blood samples were taken at regular intervals during the winter and summer seasons from both groups for the determination of thyroxine concentrations. Pen conditions created a significant (P ≤0,05) rise in thyroxine values of the beef heifers. Breed effect was also significant (P ≤0,01). The thyroxine levels of Afrikaner heifers showed the greatest increase during penning (P ≤0,01) followed by those of the Bonsmara (P ≤0,05) and Shorthorn heifers. During the subsequent summer season the influence of penning was considerably reduced, with only the Afrikaner heifers having significant (P ≤0,05) higher levels under pen than under veld conditions. Extending the penning period from 7 to 14 weeks did not result in any additional differences in thyroxine concentrations between groups.