A urea assay or analysis process and reagents therefor, in which the assay involves a reaction between a sample of biological fluid and an acidic reagent solution of o-phthaldehyde and a chromogenic compound; the chromogenic compounds include certain substituted benzene compounds, certain substituted naphthalene compounds, certain aminopyrimidines, certain substituted quinolines, certain morpholino substituted quinolines and certain morpholino substituted naphthalenes.
