The effect of current 48-hour infusion of estradiol benzoate (EB) on the dynamics of the concentration of progesterone (P) and thyroxine (T4) was studied in four heifers in the luteal stage of their cycle. The parameters were evaluated in three intervals: infusion (A), post-infusion (B) and "uninfluenced" interval (C). In the course of infusion the concentrations of P in three animals had a markedly descending tendency. The P concentration increase in the 36th hour of infusion was maintained also during the post-infusion interval. The decrease in P in the third, "uninfluenced", interval was replaced by a new rise towards the end. One heifer showed an increase in P concentrations at the beginning of EB infusion. A decrease was recorded towards the end of the interval. The concentration decreased in the post-infusion period. A comparatively high concentration of P in the "uninfluenced" interval was again replaced by a drop towards the end in the 286th hour. The concentration of T4 in three heifers had a descending tendency in the infusion and post-infusion intervals. The decrease in the infusion interval was more pronounced. In the "uninfluenced" interval the T4 concentrations increased step by step. In one heifer the concentration of T4 slightly increased during infusion. A decrease was recorded in the post-infusion and "uninfluenced" interval. Rectal examination in the 172nd hour revealed the original corpus luteum in three animals and the production of the corpus luteum in the same ovary was found in this period in one heifer. In the 88th hour this animal showed clinical manifestations of oestrus. On the basis of the dynamics of the studied hormones and clinical studies, the thyroid hormones can be regarded as an important secondary factor involved in the permanent cyclic changes in reproduction process.