A very small percentage, below 10%, of Amaranthus retroflexus seeds were germinated at 25°C in the light and thus the seeds were considered to be primary dormant. Nitric oxide (NO) applied for only 5 h stimulated germination of these non-deep physiologically dormant seeds. Likewise, ethephon, ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC) and gibberellic acid (GA 3) induced dormancy release. The stimulatory effect of NO, ACC, and GA 3 on germination was associated with increased ethylene production prior to radicle protrusion. The nitric oxide scavenger, cPTIO, antagonized the stimulatory effect of GA 3 , ethephon, and ACC on seed germination, indicating that the presence of endogenous NO is required for dormancy release by these compounds. This scavenger inhibited both germination and ethylene production stimulated by NO and GA 3. An inhibitor of ethylene binding to its receptor, 2,5-norbornadiene (NBD), antagonized the beneficial effect of ethylene and NO, indicating that ethylene action is necessary for the appearance of seed response to these gases. The ACC synthase inhibitor, aminoethoxyvinylglycine (AVG), and the ACC oxidase inhibitor, a-amino-isobutyric acid (AIB), strengthened the effect of NBD on the germination of NOpretreated seeds. Induction of germination of dormant seeds by NO, ethephon or GA 3 was associated with initiation of the cell cycle prior to radicle protrusion. The data presented indicate NO crosstalk with ethylene and GA 3 in regulating dormancy release in A. retroflexus seeds. Keywords Amaranthus retroflexus Á Ethylene Á Germination Á GA 3 Á Nitric oxide Á Primary dormancy Abbreviations ACC 1-Aminocyclopropane-1-carboxylic acid ACO ACC oxidase ABA Abscisic acid AIB a-Amino-isobutyric acid AVG Aminoethoxyvinylglycine cPTIO 2-(4-Carboxyphenyl)-4,4,5,5tetramethylimidazoline-1-oxyl-3-oxide ETH Ethephon GA 3 Gibberellic acid GA s Gibberellins NBD 2,5-Norbornadiene NO Nitric oxide