The effect of cellular stress factors, the hyperthermia and the acidification of culture medium, on mechano-growth factor (MGF) synthesis by murine myoblasts in culture was investigated. Hyperthermia was shown to stimulate MGF expression both in primary myoblasts and in differentiated multinuclear myotubes. The induction of MGF synthesis peaked at 40 degrees capital ES, Cyrillic, with some activation at 39 and 41 degrees capital ES, Cyrillic. Decrease of culture medium pH stimulated MGF expression with a maximum at pH 6.3. Hydrocortisone eliminated induction of MGF synthesis completely both under hyperthermia and acidification conditions.