Specific binding of radiolabelled FSH and LH to rat ovaries was demonstrated at the age of 7 days. However, when the biological response to LH and FSH was monitored by cAMP production in vitro, the FSH response appeared earlier than that of LH, on days 4 and 7, respectively. Cholera toxin stimulated cAMP production even in fetal ovaries, suggesting the presence of functional post-receptor machinery of cAMP production. Hence, the appearance of the functional gonadotropin receptor probably plays a key role in the onset of postnatal ovarian steroidogenesis. To test the effect of gonadotropin suppression during postnatal ovarian development, a potent GnRH antagonist was administered to neonatal animals between days 1-6 or 1-9 of life. The ovarian responsiveness to FSH developed even in the absence of normal gonadotropin levels, but that to LH was suppressed after the longer antagonist treatment. The temporal relationship between the onset of LHR gene expression, i.e. transcription, and translation to functional receptor protein was thereafter investigated using the reverse transcriptase-polymerase chain reaction (RT-PCR) technique. The measurements revealed the existence only of truncated versions of LHR mRNA in the fetal ovary from day 17 of gestation up to day 7 of postnatal life. With the onset of the receptor function around day 7, also larger mRNA transcripts, corresponding to the full-length receptor protein appeared. Our findings suggest that the LHR gene may be constitutively expressed in the ovary and a change in the alternative splicing pattern may cause the onset of translation of a functional receptor protein.