Abstract(dl) 1,2,3,4‐Tetrahydro‐α‐naphthoic acid, selected for investigation on account of the fact that we deemed it to meet especially well the spatial requirements for active compounds, proved to be very active in the pea‐test, After resolution of the racemate (with (−)menthylamine) it appeared that its activity for the greater part is due to that of the (−)acid (activity comparable with that of α‐naphthylacetic acid), the (+)acid being weakly active only.(dl)8‐Methyl‐1,2,3,4‐tetrahydro‐α‐naphthoic acid shows the same activity as the unsubstituted acid. 1,2,3,4‐Tetrahydro‐β‐naphthoic acid and its 8‐methyl‐derivative are inactive. A possible explanation for this inactivity is discussed.As “open” forms of these partially hydrogenated naphthoic acids a number of (dl)α‐alkyl‐phenylacetic acids was investigated, of which (dl)allyl‐phenylacetic acid proved to be the most active one. The resolution of this acid was carried out by means of (+) and (−)α‐phenyl‐ethylamine. Here the (−)acid is practically inactive, the whole of the activity being due to the (+)acid).There are indications that the active (−) 1,2,3,4‐tetrahydro‐α‐naphthoic aci