We measured expression of the oxytocin gene in the supraoptic nucleus (SON) during pregnancy, parturition and lactation to examine its relationship to states of accumulation or depletion of oxytocin stores and to conditions of strong excitation of oxytocin neurons. The primary transcript (heterogeneous nuclear RNA, hnRNA) of the oxytocin gene was measured using a 3H-cDNA probe against intron 1 for in situ hybridisation. Autoradiographs of the SON showed the hnRNA as discrete clumps of silver grains within the nucleus of each neuron. The number of cells expressing oxytocin hnRNA did not change during pregnancy but increased during parturition; 10-day lactating animals showed similar increases. Oxytocin mRNA was also measured by in situ hybridisation using a 3H- or 35S-labelled oligonucleotide probe against exon C: hybridisation was seen over the cytoplasm of supraoptic neurons, but no differences were measured between virgin, mid-pregnant, preparturient, parturient or 2-day lactating rats. The data suggest that enhanced oxytocin gene transcription is not necessary to increase oxytocin stores in pregnancy. However, acute stimulation of magnocellular oxytocin neurons at parturition, which strongly increases neuron activity and secretion, results in a rapid increase in the number of cells expressing oxytocin hnRNA, and increased expression is sustained in lactation.