ENThe so-called pacification Seynl (Diet) held in 1673 passed a resolution to hold every third diet in Grodno. From that year, the Commonwealth had two capitals - Cracow and Vilno - and two towns with parliamentary sessions - Warsaw and Grodno. The Union of Lublin marked the beginning of a custom to hold the diets in a royal residence: this was the result of a conception of the political system of the Commonwealth in which the monarch, alongside the Senate and the land deputies, was one of the three estates present in the Seym. The castle in Grodno was built in the Middle Ages by the Lithuanian dukes, and enlarged and fortified upon the initiative of Duke Witold at the turn of the fourteenth century. King Stefan Batory rebuilt a part known as the Old Castle in ca 1580, and erected a Late Renaissance palace which was designed by Santi Gucci, possibly with the assistance of Rudolfino da Camerino, a fortifier. The castle became one of the Lithuanian residences of the Polish kings, and was particularly frequented by Władysław IV. Both castles, Old and New, were destroyed during the Swedish and Muscovite invasions of Poland in the years 1655-1657. In 1673 it was decided to rebuild the castle and to adapt it for purposes of the diets. The first, in effect inaugurational diet in Grodno, chaired by Franciszek Sapieha, the Equiry of Lithuania, began its sessions on 15 December 1678. Hieronim Piasecki, the Marshal of Slonim, was requested by Krzysztof Pac, the Grand Chancellor of Lithuania, to supervise the rebuilding of the castle, which was initiated already in the same year.The reconstruction resulted in the addition of a royal apartment, two large chambers for the plenary sessions of the Senate and the Chamber of Deputies, as well as auxiliary rooms such as the chancery, a conference hall etc. The Old Castle was destroyed at the beginning of the eighteenth century. So far, no plans dating from the period prior to the catastrophe have been traced. The inventory from 1680, containing a description of the castle immediately after the reconstruction conducted by the Pac family also did not survive. In this situation, the establishment of the dimension of the rebuilding remains an extremely difficult task. An analysis of other archive documents indicates that a new wing, or rather a separate edifice intended for the sittings of the Seym was erected in the Old Castle in the years 1673-1678. It contained chambers for the deputies and senators, which were connected with the royal apartment in Batory’s palace by means of a corridor-gallery. A spacious chamber in the palace fulfilled the function of the royal audience hall. Both Seym chambers were probably decorated with stucco ornaments whose main motif could have been, similarly to the Royal Castle in Warsaw, the glorification of the knights’ estate in the Chamber of Deputies, and of the republican principles of the political system of the Commonwealth in the senatorial hall.The con- tentual programme in Grodno was possibly conceived by Chancellor Pac, and the form of the decorations could have resembled that of the Bernardine Lorrain chapel in Grodno from the end of the seventeenth century. The further fate of the Seym building in the Old Castle appears to be rather unclear. Probably some sort of a catastrophe took place between 1688 and 1690, and its aftermath was liquidated under the supervision of the sub- camerarius of Lida. The size of the devastation and the ensuing restoration conducted in 1690 remain unknown. Apparently the losses were considerable, and one cannot even exclude a complete reconstruction of the Pac building. At any rate, the next Seym, in accordance with the Lithuanian alternate, sat in Grodno from 9 January up to 11 February 1693. After the death of Jan III Sobieski and the election in 1697 of King August II, the Lithuanian alternate ceased to be applied. Out of a total of nine diets convened by the new monarch, eight took place in Warsaw and one in Lublin. A parliamentary session was held in Grodno as late as 1718. It seems that following the Northern War the castle was no longer regarded suitable either as a royal residence or for the Seym. Tnis is why in 1718 Sapieha palace, located in the market square, was chosen for parliamentary purposes and, as the residence of August II, it was adapted for its new functions.