The article deals with Czech toponyms with the neuter ending -o, occurring almost exclusively in Southwest Bohemia. These neuter forms are regarded as (mostly unofficial) variants of masculine names. These forms are attested both in non-settlement names (mostly field names) and settlement names. Especially the settlement names data are rather rich; they include unofficial variants of toponyms and their historical records (some of the historical records are very old, beginning as early as in the 12th century). The area of occurrence of toponyms of this type has been compared to the dialectal area of the indeclinable form of possessive adjectives ending in -ovo found in Southwest Bohemia; the correspondence of these two areas is striking and can hardly be coincidental. According to the author’s hypothesis, the change of the toponyms ending in -ov and -ín to -ovo and -ino started in the historical period in which the original possessive function of the names and their connection with possessive adjectives was still transparent. Based on the toponymic evidence, it seems that the Southwest Bohemian dialectal phenomenon of possessive adjectives ending in -ovo must be extremely old, reaching as far back as the 12th century.