We have compared the expression patterns in transgenic mice of bovine oxytocin constructs consisting of the 0.9 kilobase pair (kbp) structural gene flanked by varying lengths of upstream and downstream sequences. Over 200 offspring were derived from fertilized one-cell mouse eggs injected with construct bOT6.5, which consists of 3 kbp of upstream sequences and 2.6 kbp of downstream sequences. However, no transgenic founders were identified. In parallel experiments with other constructs, 30% of pups carried integrated copies of the injected transgene DNA. It therefore appears that bOT6.5 is toxic to mouse embryos. As previously reported (Ang et al., 1991), bOT, consisting of 2.6 kbp of downstream sequences and 0.6 kbp of upstream sequences, was expressed in lung and in testicular Sertoli cells, but no expression was detected in the hypothalamus. bOT3.5, which consists of 0.6 kbp of upstream sequences and 1.9 kbp of downstream sequences, retains testis and lung expression, but, surprisingly, is also expressed in the hypothalamus. These data suggest that the 0.7 kbp of downstream sequences that are present in bOT, but which are absent from bOT3.5, contain elements that mediate the repression of hypothalamic expression. The activity of this repressor must be itself overcome in the normal genomic context of the bovine OT gene. Within the hypothalamus, in situ hybridisation analysis has revealed expression of bOT3.5 in the supraoptic nucleus (SON) and paraventricular nucleus (PVN), but not in the suprachiasmatic nucleus (SCN). In parallel with the response of the endogenous murine OT RNA, 7 days of salt-loading resulted in a significant increase in the level of transgene RNA in the SON. In the PVN, neither the endogenous OT RNA nor the transgene RNA responded significantly to salt-loading. Transgene RNA levels in the hypothalamus have also been shown to be elevated during late pregnancy and lactation.