[holding answer 15 March 2007]: I assume that the question refers to Article 30 of the Treaty of Rome, which was Article 36 prior to the current Treaty.
I am aware of two rulings by the European Court of Justice in cases relating to the export of live animals in which Article 30 (then Article 36) was invoked. Case C-5/1994, ruled on 23 May 1996, relates to a UK ban on exporting sheep to Spain because of concerns over welfare conditions in Spanish abattoirs. Case C-1/1996, ruled on 15 July 1997, was a case brought by Compassion in World Farming on banning the export of calves destined for veal crates. In both cases, the European Court of Justice made clear that a European Union member state may not ban or limit a trade if it is subject to harmonised rules that must be complied with equally by all member states.