Mystirio sau, Sacrament, sau, Taine 2, de în ceale 7, Botezul și S[fân]t[ul] Mir (= MYS) is a liturgical handbook for priests printed in 1651 in Târgoviște (Wallachia) at the initiative of Metropolitan Stephen I. This book of small dimensions comprises an introductive guide of sacramental theology for two Sacraments of Christian initiation, namely the Holy Baptism and Holy Chrismation, together with the description of their ordo and other prebaptismal and postbaptismal rites, the Canons of the Apostles and Holy Fathers for the Sacrament of Baptism, the ordo when in extreme urgency occasion arises to give communion to a sick person, a prayer for one who has eaten something defiling and instructions for the confessor priest. Although the prayers, liturgical formulas and scripturistic lectures are printed in Slavonic, the teachings and the rubrics are translated into Romanian language. In this paper the author analyses the liturgical rites described in MYS and, comparing it with other contemporary liturgical documents, demonstrates that while some parts in MYS were taken from an Slavonic Euchologion printed at Câmpulung (Wallachia) in 1635, the primary source of this book is the Kievan Trebnik of Peter Mohyla (1646).