cDNAs encoding the precursor molecule of the turkey LH β subunit (tLHβ) were cloned from a turkey pituitary cDNA library. The nucleotide sequence of the longest of two different tLHβ cDNA clones contained 592 bp, and included 23 bp of the 5' untranslated region (UTR) and 92 bp of the 3' UTR in addition to a 477 bp open reading frame that encoded a 39 amino acid leader polypeptide and a 120 amino acid mature apoprotein. Turkey and chicken LHβ sequences shared approximately 92 and 93% nucleotide and amino acid sequence similarities respectively. Northern blot analysis of total cellular anterior pituitary RNA showed that an approximate 800 base transcript hybridized to a 32P-labelled tLHβ cDNA probe. The gonadotrophin-releasing hormone (GnRH)- and prolactin (PRL)-regulated expression of LH and PRL in dispersed pituitary cells was determined by Northern blot analysis of tLHβ and PRL steady-state mRNA levels and by RIA analysis of secreted LH and PRL. GnRH-treated cells showed increased levels of both tLHβ mRNA and secreted LH, whereas mRNA and secreted levels of PRL did not change significantly. Cells treated with PRL showed lower levels of tLHβ and PRL mRNA as well as decreased release of LH and PRL. When cells were treated with both PRL and GnRH, increases in tLHβ mRNA and secreted levels of LH observed with GnRH alone were negated, whereas the decreases in mRNA and secreted levels of PRL observed with PRL alone were abrogated. These findings suggest that PRL can down-regulate tLHβ gene expression and spontaneous release of LH as well as autoregulate PRL gene expression and spontaneous release of PRL, while GnRH appears capable of modulating the effects of PRL-regulated LH and PRL gene expression and spontaneous release.