Phospholipid methylation in Leydig cells from desensitized rats was studied. The incorporation of L-[methyl-3H]methionine into phospholipids in intact Leydig cells decreased when animals were injected with a single high dose of human chorionic gonadotrophin (hCG). This effect was detected on the first day after hCG injection and remained up to 12 days after treatment. The inhibition was not due to a reduced uptake of L-[methyl-3H]methionine. A decreased phospholipid methylation with unaltered phospholipid methyltransferase activity was observed on days 1, 6 and 12 after the hCG. On day 3 after hCG injection, phospholipid methyltransferase activity and phospholipid methylation in intact Leydig cells were both inhibited by 40%. Also, a minimal amount of LH free receptors and the lowest number of total receptors was observed at this time. Thus, a relationship between the reduced enzymatic activity and the maximal decrease in LH surface receptors is suggested. In addition, the decreased incorporation of L-[methyl-3H]methionine into phospholipids on days 1, 6 and 12 after hCG injection, could be associated with other cellular changes related to the desensitization process.