We have examined the use of RNA interference as a means of downregulating gene expression and provide the first comparison of shRNA and artificial miRNA constructs for transgenic livestock. Several in vitro assays were performed to identify the most effective RNAi constructs. shRNA and miRNA constructs achieved significant downregulation of two porcine target genes: the milk whey protein beta-lactoglobulin and the tumour suppressor p53. Results of different assays were, however, sometimes at variance, indicating that no one assay can be relied upon to predict the effectiveness of an RNAi construct. Our findings are that screening of RNAi constructs is most informative if carried out in primary cells that express the target gene and are competent for somatic cell nuclear transfer. Importantly, the use of miRNA constructs makes tissue specific gene knockdown in large animals a realistic possibility.