The purpose of this study was to re-assess the classification in vivo of a series of (-)-normetazocine derivatives as kappa-opioid agonists. From their ability to inhibit contractions of the electrically stimulated rabbit vas deferens, the (2"S)-N-(2"-methoxypropyl) and N-(2"-methoxyisobutyl) derivatives were identified as full agonists. Several partial agonists were discerned, including the (2"S)-N-tetrahydrofurfuryl derivative, Mr2034. (+)-tifluadom, (+/-)-bremazocine, (-)-ketazocine and (+/-)-U-50,488 were used as reference compounds. Structure-activity relationships are discussed.