The Ca++ antagonist effects of new drugs derived from monosaccharides were tested in the rat duodenal muscle preparation in vitro. The structure-activity relationship shows that: 1. The monoacetonide products (nR-O-MAG and nR-S-MAG) with alkyl chain from 8 to 9 C atoms induce a maximal reduction of muscular tonus and contraction. 2. The inhibitory effect rapidly decreases when the alkyl chain has a number of C atoms smaller than 7 or larger than 9. 3. The diacetonide (DAG) and acetonide (G) products induce an inhibition of less extent. 4. The Ca++ antagonist effect is very slightly changed by the type of heteroatom (O or S) linking the alkyl chain to the monosaccharide. 5. The type of monosaccharide affects the Ca++ antagonist activity.