An experimental program was conducted in St. Louis for the further development and testing of a practical, multipurpose urban diffusion model (APRAC-1A) for carbon monoxide to determine whether earlier (1970) findings in San Jose, California could be generalized to larger cities with highly developed urban cores. Two adjacent downtown street canyons were instrumented to obtain measurements of CO concentration at 30 points and winds at eight. Helicopter-and van-borne instrumentation was used to supplement data collected with the automated street-canyon instrumentation system. The model was revised to improve the specification of atmospheric stability and small-scale street-canyon effects. The distribution of CO in the street canyon indicates the presence of a single-cell, helical circulation in the deep St. Louis street canyons under cross-street, roof-level flow conditions; the same pattern was found in the shallow San Jose canyon.