The domestic pig, Sus scrofa, builds a maternal nest in the day before parturition. A model for porcine nest building has been established, in which exogenously administered prostaglandin (PG)F(2alpha) is used to induce nesting behaviour in cyclic, pseudopregnant and pregnant pigs. This experiment was designed to examine the effect of PGF(2alpha) on the preferences of non-pregnant gilts for pens bedded with straw compared with bare pens. Ten 6-month-old nulliparous female pigs (gilts) were tested in an arena, which consisted of four pens (1.8mx1.7m), a neutral area (1.5mx3.4m) and a start area (1.5mx3.4m). Two of the pens contained 2kg of fresh straw and the remainder of the testing arena was devoid of straw. On the first day of testing half of the pigs were given a control intramuscular injection of 3ml 0.9% saline and the remainder were given an intramuscular injection of 15mg PGF(2alpha) and their behaviour scored for 1h after treatment. On the following day the treatments were reversed, such that each pig was given both treatments (saline or PGF(2alpha)). There was no significant effect of the order of treatment on behaviour. After saline-treatment the pigs spent most of their time in the pens containing straw (59%) and the least amount of time in bare pens (5%). In the straw pens, saline-treatment induced bouts of oronasal contact with straw of a relatively long duration (11-100s), which we interpret as foraging. In the hour after PGF(2alpha)-treatment the pigs also spent most of their time in the pens containing straw (44%) and the least amount of time in bare pens (10%), but they interacted with the straw in a markedly different way. PGF(2alpha)-treated pigs displayed bouts of oronasal contact with straw of a relatively short duration (2-10s) which, together with high frequencies of pawing at straw, lifting and carrying straw in the mouth, we interpret as nest building behaviour. Superimposed on this is the finding that gilts spend more time in the neutral areas after PGF(2alpha)-treatment than they did after saline-treatment. PGF(2alpha)-treated pigs spent most of their time engaged in nesting behaviour within the straw pens but they also gathered and deposited straw in different areas of the test arena (neutral and start areas); behaviours not seen after saline-treatment. We conclude that pigs generally prefer a pen containing straw bedding to a bare pen but that PGF(2alpha) alters the way they interact with straw, inducing behaviour similar to prepartum nest building.