Changes in the level of cytosine methylation were assessed by HPLC in mechanically stressed Bryonia dioica. Rubbing young internodes. which results in growth inhibition and ethylene production, induced a rapid and transient decrease of cytosine methylation in DNA. The level of cytosine methylation was about 25% in young internodes and dropped to nearly 0% in less than 1 h before increasing to the normal level within 3 h. A decreasing gradient of DNA methylation occurred naturally along the plant, from the apex to the base. The pool of S‐adenosylmethionine was also measured in order to establish a possible relationship between ethylene metabolism and DNA methylation. The role of DNA methylation in gene regulation is discussed and different mechanisms for methylation are considered.