The ontogeny of expression of the LH receptor (LHR) gene was studied in rat testis between day 12.5 of fetal life and adulthood. Specific hybridization of testicular mRNA with a LHR cRNA probe encoding the extracellular domain of the receptor was found from day 16.5 of fetal life onward in Northern hybridization. Transcripts of 6.8, 4.2, and 2.7 kilobases were present at all ages, and a 1.8-kilobase species was present mainly in the adult testes. Hybridization was most intensive in day 21.5 fetuses, decreased after birth, and increased again by adulthood. The LHR mRNA was also analyzed by the reverse transcriptase-polymerase chain reaction technique, with primers multiplying either the full-length LHR mRNA or its extracellular domain. The specificity of the DNA species generated was verified by Southern hybridization using a nested 32P-labeled oligonucleotide. The results indicated that a truncated mRNA form, encoding the extracellular part of LHR, appears 1 day before the full-length LHR mRNA, i.e. on fetal days 14.5 and 15.5, respectively. This is in striking contrast to the rat fetal ovary, in which a difference of more than 10 days is found in the appearance of these two LHR mRNAs (17.5 days of fetal and 7 days of postnatal age, respectively). The appearance of the full-length LHR mRNA coincides in both sexes with the developmental onset of LHR binding observed in earlier studies. In situ hybridization using an antisense cRNA probe demonstrated that the LHR mRNA was confined to Leydig cells at all fetal and postnatal ages studied. In conclusion, there is good correlation in the developing rat testes between the onset of LHR gene expression and LHR binding, as observed in earlier studies. The findings in the fetal testis are at striking variance with those in the ovary, which starts expressing the extracellular domain of the LHR mRNA at roughly the same age as the testis. However, the appearance of full-length LHR mRNA and the functional receptor are delayed until day 7 postpartum.