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Ummmms and aaahhhs are not the only co-partners of kissing, for a passionate lip-lock unleashes a chain of chemical changes that really turn your head, claims a new study.

As Valentine's Day approaches, Wendy Hill, professor of psychology at Lafayette College, Pennsylvania has taken the opportunity to shed light on that most basic of all human expressions of love - the smooch.

In her study, Hill has found that a meeting of lips can spark a complex chemical surge into the brain that makes a lover feel excited, happy or relaxed.

Also, it is being speculated that the hormone release may be triggered directly by an exchange of sexually stimulating pheromones in the saliva.

"This study shows kissing is much more complex and causes hormonal changes and things we never thought occurred," The Times quoted her, as saying.

"We tend to think more about who we are kissing and how it feels, yet there are a lot of other things happening," she added.

To reach the conclusion, the research team looked at the impact of kissing on levels of two hormones, oxytocin and cortisol, in 15 male-female couples before and after holding hands and before and after kissing.

Oxytocin is known to be involved in social bonding so the researchers predicted that its levels would rise, while cortisol, a stress hormone, would fall. The results showed cortisol levels fell in both sexes, although oxytocin levels rose in men but fell in women.

Detailed results will be published at the American Association for the Advancement of Science's annual conference in Chicago this week.