Twenty-four immature hybrid gilts (Large Polish x Polish Landrace) were randomly divided into three groups in order to determine zearalenone (ZEA) and α-zearalenol concentrations in blood after feeding them with feeds containing low doses of crystalline zearalenone (groups II and III, 200 and 400 μg kg^-1 body weight, respectively). The mycotoxin was administered in gelatine capsules once a day during the morning feeding (at 6.30) for seven consecutive days. On the first day of the experiment, 5.5 h after feeding, the concentration of ZEA was the highest in group II (9.43 ± 1.32 ng ml^-1), and the concentration of α-ZEA was the highest in group III in the second hour of the experiment (24.13 ± 5.98 ng ml^-1). Mean concentrations of ZEA were: 1.54 ± 0.16 up to 9.92 ± 0.80 ng ml^-1 (II) and 2.34 ± 0.33 up to 10.90 ± 0.93 ng ml^-1 (III), respectively, whereas mean concentrations of α-ZEA were: 2.86 ± 0.23 up to 8.11 ± 0.60 ng ml^-1 (II) and 4.48 ± 0.22 up to 17.79 ± 2.19 ng ml^-1 (III), respectively. It was found that administering a LOAEL (Lowest Observable Adverse Effect Level) dose of ZEA per os caused weakly manifested symptoms of hyperoestrogenism, whereas swelling and reddening of the vulva were much stronger at a dose double the LOAEL dose. There was no distinct positive correlation between the level of zearalenone and α-zearalenol in blood and the occurrence of the external symptoms of hyperoestrogenism.