In Trial 1, seven Large-White × Landrace sows were initially housed in either stalls (n-4) or straw pens (n=3) during gestation. On Day 107 of pregnancy, each sow was placed into a pen in which food was delivered every 2 h and which had operant access to a straw area (S) and a control area (C). Two days before farrowing, sows spent 71% of their time in S and made significantly more entries into S than C (19.6 vs. 4.4). On the day before farrowing, entries into S increased to 37.4 and all the sows built nests and farrowed in S. Straw was reinforcing but did not appear to be as important as feed. In Trial 2, five sows initially housed in straw pens, were each placed, on their Day 100 of pregnancy, into a pen which had operant access to a straw area (S) and a feed area (F). The number of entries made into S and F were, respectively, 17 and 21.4 when cost of entry was low (one panel press) and 2.6 and 11.4 when the cost was high (a mean of 150 panel presses) 2 days before farrowing and 16.4 and 17 on the day before farrowing when the cost was also high. It was concluded that the importance of straw approached that of feed, particularly during the 24 h before farrowing when sows were motivated to nest-build.