This article is from the Chihuahua Breed FAQ, by firstname.lastname@example.org (Melinda 'Bob' Casino) with numerous contributions by others.
Yes, they are high-strung dogs. I define "high-strung" as follows:
barks easily, does not adapt easily to change in environment, is
suspicious of strangers and will growl at them, and flips in circles
and jumps around when excited (like when you come home after a
five-minute absence, for instance).
However, with the person that they have bonded with (i.e., their
master[s]), they do not display most of these characteristics;
infact, they display radically different personalities. Chihuahuas
are truly the "Jekyll-and-Hyde" of dog breeds: your friends will see
the worst side of them and never believe you when you tell them that
your Chi is really a gentle, sweet-natured dog.