The MAPK cascade in Xenopus oocytes exhibits an all-or-none, ultrasensitive response, which is believed to result from a positive feedback loop. We tested the hypothesis that an ultrasensitive response may be generated for MAPK response in absence of feed-back loop, through other regulation motifs, and seek for experimental conditions where MAPK could be activated in absence of feedback. Here we describe a context where 1,10‑Phenanthroline slowly, but strongly, activates MAPK while it impairs protein synthesis in a zinc-dependent manner, abolishing any feedback loop. The induced‑MAPK response was found to be strongly ultrasensitive. This behavior may reveal a regulation motif akin to a feed-forward loop acting in vivo.