Abstract Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) result from the assembly of a common subunit α and a unique subunit β, expressed in the same cell by single, structurally-related genes. In order to compare the intrinsic stability of the α, LHβ and FSHβ mRNA transcripts, we used cultured rat pituitary cells incubated in presence of actinomycin D. Hybridization with 32P-labelled rat cDNA probes showed that the cell content of all three mRNAs decreased with time, but at different rates. Apparent half-lives, estimated as the time necessary to observe a 50% mRNA decay, were 1.0 ± 0.13 h for FSHβ, 6.5± 0.25 h for α and 44 ± 0.5 h for LHβ stability thus exhibiting an inverse relation to the sizes of the corresponding mRNAs (~ 1700, 800 and 700 nucleotides, respectively). Northern analysis revealed that the decline in mRNA abundance was associated with a progressive decrease in the length of mRNAs, most clearly visible for α and LHβ. For the most stable LHβ mRNA, shortening was apparent as early as 2 h after exposure to actinomycin D thus preceding neatly the decrease in amount starting at about 10–12 h. In vitro RNase H digestion demonstrated that shortening resulted from a reduction of the length of the poly(A) tract. These data establish that the three mRNAs coding for gonadotropin subunits have different stabilities although they share substantial homology. Diversity in size and sequence essentially resides in untranslated regions in which, we suggest, specific motifs and protein factors may interact to determine mRNA stability. The presence and length of the poly(A) stretch would also play a role.