SUMMARY.–The paired costovertebral organs of the hamster are presented as an assay for distinguishing direct topical activity from indirect activity subsequent to systemic absorption. Topical chlormadinone acetate and its chemical modifications 19‐nor‐chlormadinone acetate, Δ1−chlormadinone desacetate and Δ1−chlormadinone−16–17 acetonide were without demonstrable effect. Cyproterone acetate and Δ1−chlormadinone acetate showed questionable activity which does not seem to be primarily confined to a direct