It has been shown earlier that LH decreases the oxygen consumption of the rat cumulus oophorus cells, i.e. the granulosa cells immediately surrounding the follicular oocyte. The decrease occurs in the preovulatory period at the time when the oocyte resumes and completes meiosis. This study examines in more detail the time-course of LH-induced inhibition of cumulus respiration. The results show that when isolated cumuli were exposed to LH during in vitro culture a significant inhibition of respiration occurred within one hour. When follicle-enclosed cumuli were exposed to LH, in vivo or in vitro, the decrease in respiration was not evident until 4 hours after the hormone exposure. By this time oocyte maturation was already in progress. The results suggest that the decrease in cumulus respiration may not be directly linked to the onset of oocyte meiosis.