Musth or must (from Persian, lit. 'intoxicated') is a periodic condition in bull (male) elephants characterized by aggressive behavior and accompanied by a large rise in reproductive hormones.

Testosterone levels in an elephant in musth can be on average 60 times greater than in the same elephant at other times (in specific individuals these testosterone levels can even reach as much as 140 times the normal). However, whether this hormonal surge is the sole cause of musth or merely a contributing factor is unknown.

Scientific investigation of musth is problematic because even the most placid elephants become violent toward humans and other elephants during musth.

Musth differs from rut in that the female elephant's estrus cycle is not seasonally linked, whereas musth most often takes place in winter. Furthermore, bulls in musth have often been known to attack female elephants, regardless of whether or not the females are in heat.