We have shown previously that expression of 46 and 54 kDa human myometrial Gαs protein isoforms is increased during gestation and then subsequently decreased during labour. These proteins appear to be coded for by Gαs-Small (with a serine residue at position 72) and Gαs-Large (with a serine residue at position 87) mRNA splice variants respectively. In the study presented here we have used a Gαs cDNA template to generate [32P]cytidine cRNA ribo-probes for use in RNase protection assays, so as to measure total myometrial Gαs mRNA levels in relation to the pattern of expression of Gαs mRNA splice variants during pregnancy and labour. We report that total levels of human myometrial Gαs mRNA remain similar in non-pregnant and pregnant women but are substantially reduced during parturition. Our data also provide strong evidence that alternative splicing of Gαs precursor mRNA has a primary role in regulating expression of Gαs protein isoforms during pregnancy and labour. The inclusion of an additional serine codon in Gαs mRNAs during pregnancy involves a switch in alternative splicing pathways. We speculate that this switch may be due to a change in specificity of splicing factors that are modulated during pregnancy and labour.