The present invention provides isotopically labeled oxazinocarbazole derivatives having a ring connecting position 8 (C-8) and position 9 (N-9), and more specifically, provides compounds of formula (I) wherein R₁, R₂, R₃ and R₄ are described herein. ##STR1## These compounds are useful in the diagnostic analysis of diseases wherein modulation of 5-HT activity is desired.