In order to investigate the structure-activity relationships of brassinosteroids, fourteen (22R,23R,24S)-28-homobrassinosteroids with modifications in rings A and B were synthesized from (22E,24S)-3β-hydroxy-5α-stigmast-22-en-6-one (3a). The (22R,23R)-vicinal diol was introduced by epoxidation of the trans C-22(23) double bond, followed by trans ring-opening of the resulting epoxides by HBr–AcOH, and then an inversion reaction at the carbon bearing bromine by acetoxy anion to give (22R,23R,24S)-3β,22,23-triacetoxy-5α-stigmastan-6-one (8a). Baeyer–Villiger oxidation of (8a) gave the two regioiso-meric 6-oxa- and 7-oxa-lactones (15a) and (16a). Introduction of a 3α-hydroxy group and C-2(3) or C-3(4) double bond was achieved by treatment of the 3β-methanesulphonate (10b), (17b), or (25b) with lithium carbonate in refluxing dimethylformamide. Further elaboration of ring A gave 28-horao-brassinosteroids, 2-deoxybrassinosteroids, 2-deoxy-4α-hydroxybrassinosteroids, and their regioiso-meric 6-oxalactone compounds. Bioassay using the rice-lamina inclination test indicated that the 3α,4α-diol analogue (23) has almost the same activity as the 2α,3α-diol (1b), and the 2-deoxysteroids (13) and (21) were also highly active, showing one-tenth the activity of the 28-homobrassinolide (1b). The following structural features of brassinosteroids are important for the high activity: (i) the (22R,23R)-vicinal diol moiety; (ii) the (24S)-methyl or -ethyl group; (iii) the 7-oxalactone or 6-oxo functionality in the B-ring; (iv) a 3α-hydroxy group, 2α,3α-vicinal diol, or 3α,4α-vicinal diol; and (v) an A,B-trans-fused ring junction.