1. Helminthosporic acid analogs Helminthosporic acid (HA, 1) and its related compounds have been known to show gibberellin (GA)-like activites in a-amylase induction and elongation of rice shoots. These activities were suggested some roles of structural resemblance between HA and the C/D ring system in GA. We have made analogs of HA in order to confirm structural factors for the GA-like activities. One of the isomer of 14-nor-α-hydroxy HA was found to be very active in α-amylase induction tests. But 14-nor-α-hydroxy HA did not show the α-amylase induction at all, therefore the carbon skeleton of HA might play less important roles for this activity. 2. Abscisic acid analogs In a solution, cis-abscisic acid (ABA, 2) isomerized into the traps-isomer which is physiologically inactive, and this instability is regarded as a reason for insuitability of ABA in agricultural applications. The side chain of ABA, the 2-cis-4-trans-3-methyl-2,4-pentadienoic moiety, was replaced with various substituted phenyl groups to examine biological effects of the inflexibility of the part. Construction of the phenyl moiety was achieved by cyclizing the ionone derivatives and subsequent aromatization. Some of those analogs showed ABA-like activity in both seed germination and transpiration assays.