A series of close analogues of the potent, long-acting cardiotonic bemoradan (2a) was synthesized and examined in both in vitro and in vivo test systems. Changing the oxygen heteroatom at the 1-position of the benzoxazine ring of bemoradan to sulfur gave 4a, a more potent enzyme inhibitor and in vivo cardiotonic compound by the iv route. Intraduodenal administration of bemoradan, however, showed a superior response compared to its sulfur analogue, possibly due to oxidation of sulfur followed by a facile Pummerer rearrangement. Model studies were performed to examine the effect of the oxidation state of sulfur. Lack of a heteroatom at the 1-position, 3a (Y-590), afforded a compound with activity and potency very similar to those of bemoradan while the 1-selena compound gave a much less potent analogue 5. Analogues having a methyl group on the 4-nitrogen (2b, 3b, and 4b) were less potent than the desmethyl compounds, but all of these compounds have potent PDE III inhibiting activity and the ability to increase cardiac force in an anesthetized dog preparation when given iv.