The concentrations of ovarian steroids (estradiol ? E2, progesterone ? P4 and testosterone ? T) and thyroid hormones (thyroxine ? T4 and triiodothyronine ? T3) were determined in blood plasma of the domestic hen during sexual maturation and the initial period of egg lay. Blood samples were collected from Hy-Line pullets at 3 day intervals from days 87 to 144 day of life, i.e. 42 days before and 14 days after the onset of egg lay (OEL). Ovarian and thyroid hormones were measured by RIA methods. During sexual maturation an increase in ovarian steroids in the blood plasma was observed. The maximum E2 and P4 levels were recorded on day 6 and day 3 prior to OEL, respectively. In the case of plasma T level, an increase from 42 to 18 days before OEL followed by a decrease and a renewed increase from day 9 till OEL was observed. The relatively unchanged plasma level of T4 until day 9 before OEL decreased significantly just before the first oviposition while the T3 level gradually decreased between day 42 and day 9 before OEL, and then increased and again decreased from day 3 before till day 3 after OEL. During sexual maturation the following statistically significant coefficients of correlation between ovarian steroids and T3 were found: E2 vs. T3 ? r = -0.551 and P4 vs. T3 ? r = -0.373. There was no significant correlation between T and T3 or between the examined steroids and T4. The data obtained indicate that during sexual maturation of the domestic hen there is a negative relationship between the ovary and the thyroid gland.