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Alkaline rocks of prevailing sodic composition form major ring complexes and stocks along a seismically active N–S structural lineament parallel to the Paraguay river course. All these occurrences are encompassed by the Alto Paraguay Province, their rocks corresponding to the oldest known alkaline magmatic activity in the Paraná Basin, with a Middle Triassic age of 241.5 ± 1.3 Ma. Intrusive rocks ranging from silica-undersaturated (nepheline and/or sodalite syenites) to silica-oversaturated (quartz syenites and alkali granites) and their fine-grained counterparts constitute the most abundant rock-types. Clinopyroxenes, associated amphiboles and varying amounts of biotite are the main mafic minerals present. Alkali feldspar (mainly mesoperthites and subordinate sanidine and anorthoclase) and feldspathoids (nepheline and/or sodalite) are widespread. A late-stage magmatic to metasomatic/deuteric mineral assemblage is mostly typical of miaskitic to agpaitic rocks of the Cerro Boggiani complex, whereas rare accessory silicate minerals are found in peralkaline syenites of the Cerro Siete Cabezas major intrusion. Positive Nb–Ta and Zr–Hf anomalies and negative Sr, P, and Ti spikes are common features to the whole province. REE behavior shows enrichment in LREE and strong LREE/HREE fractionation with various parallel subtrends. This paper addresses petrological aspects of the origin and evolution of these alkaline rocks. • The paper reviews knowledge about the Alto Paraguay Alkaline Province (border between Brazil and Paraguay). • The main rock types vary from undersaturated (nepheline and sodalite syenites) to silica-oversaturated (quartz syenites and alkali granites) and their fine-grained equivalents. • The intrusion of the alkaline bodies took place along an N–S-oriented structure that marks the boundary of the Pampia and Rio Apa cratonic blocks. • The rocks are dated from the Middle Triassic and constitute the oldest alkaline magmatic event on the edge of the Paraná Basin. [ABSTRACT FROM AUTHOR]