The author explores the peculiarities of the functioning of parliamentarism in the Ukrainian lands during their stay in the Commonwealth. The county Sejm of the Belz Voivodeship in the 1570s is used as an example. Particular attention is paid to the activities of representatives of this region at the Toruń Sejm in 1576. As a result of the research, it is established that the Belz ambassadors were consolidating with the representatives of the Ukrainian voivodeships and the crown nobility in the main issues considered during this Sejm. Thus, the nobility demanded the establishment of the Crown Tribunal and the strengthening of the defense of the south-eastern borders of the state. Instead, the newly elected monarch insisted on the need to convene a joint movement to quell the uprising in Gdansk and adopt new taxes. Due to the different positions of the king and the Chamber of Ambassadors, the Sejm ended without the adoption of the Constitution and the Universal Order.