Looks like we can add kissing to the list of activities that promote the release of oxytocin. According to a story in that most romantic of publications, Scientific American, Wendy L. Hill and her student Carey A. Wilson of Lafayette College compared the blood levels of cortisol and oxytocin in men and women, before and after they kissed.
They expected oxytocin to go up and cortisol to go down post-liplock. Surprisingly, only the men's oxytocin levels rose. The women's actually got lower. Cortisol levels dropped after kissing in both sexes.
SciAm got their conclusion a little wrong. According to the article, Hill and Wilson hypothesize that women need more than a little kiss to get the love thang going. In fact, the researchers thought that the atmosphere of the lab might be at fault. From the abstract:
... the gender effect on changes in OT suggests that females may require a more intimate atmosphere than males in order to respond positively to a kiss.
It's also possible that blood levels don't accurately reflect the levels in the brain in this case.
Laura's Psychology Blog has all the relevant links.