A phenomenon of LTP was first observed in hippocampus in 1960s. It was found that after a tetanization (high-frequency afferent stimulation of hippocampal fibers with an electrode) the evoked response increases greatly. This increase can last for minutes, hours, days and even weeks and months. LTP is a most convinient electophysiological experimantal model for memory (and more - for associative and Hebb-rule memory).
First experiments with LTP were made in vivo, but later first in vitro experiments were made as well, using slice techinique.
LTP was observed also in cerebellum and cortex.
The phenomenon opposite to LTP is called LTD (Long-Term Depression).