Claudio Monteverdi's L'Orfeo, which premiered in the Mantuan court in 1607, is generally acknowledged as the first full-fledged opera ever written. And it is an almost perfect opera in its pacing, proportions, and control of dramatic tensions and contrasts. Monteverdi, who later laid the foundations for the Neapolitan opera that led to the flowering of the bel canto style, was keenly aware of the revolution in music drama he was helping to foment. The subject matter of the five-act opera, based on the Orpheus legend, is really about the power of music. In fact, in the prologue the allegorical figure of La Musica comes forward and obliquely declares what opera should be. Throughout L'Orfeo Monteverdi put to compelling use the newfangled recitative style that had brought opera into being. He also came up with effective instrumental colorings that vividly underscore the emotional states of the characters. In this concert presentation, believed to be the first professional revival of L'Orfeo in Chicago, Music of the Baroque will not resort to any gimmicky period instrument treatment. Instead the focus will be on bel canto singing, for which the ensemble is justifiably celebrated. Kevin McMillan, the much-touted young Canadian baritone, makes his MOB debut in the title role. Patrice Michaels Bedi, probably our town's best soprano, sings both Euridice and La Musica. Other soloists include Steven Rickards, Karen Brunssen, and William Watson. Thomas Wikman conducts. Friday, 8 PM, Grace Lutheran Church, 7300 W. Division, River Forest. Sunday, 3 PM, United Church of Hyde Park, 1448 E. 53rd. Monday, 8 PM, St. Pauls United Church of Christ, 2335 N. Orchard. Wednesday, 8 PM, First United Methodist Church of Evanston, 1630 Hinman, Evanston. 986-3236.