DURING the past two years I have had to spend periods of several weeks on a remote island in comparative isolation. In these conditions I noticed that my beard growth diminished, but the day before I was due to leave the island it increased again, to reach unusually high rates during the first day or two on the mainland. Intrigued by these initial observations, I have carried out a more detailed study and have come to the conclusion that the stimulus for increased beard growth is related to the resumption of sexual activity.