The role of contraction in blastocyst hatching was determined by time-lapse videomicrography in mouse blastocysts whose hatching ability had been suppressed by soybean trypsin inhibitor (STI). The hatching rate of blastocysts developed from morulae in a medium containing STI at a concentration of 1.0 mg/ml (STI-treated blastocysts) was 17.2%, which was significantly lower than the 63.9% for blastocysts developed from morulae in a medium without STI (non-treated blastocysts). Over the span of 32 hrs after blastocoel formation, the number of strong contractions was similar in STI-treated and non-treated blastocysts, but the total number of contractions and the number of weak contractions (less than 20% volume reduction) were significantly smaller in STI-treated blastocysts (4.22 and 2.94 times) than in non-treated blas-tocysts (5.80 and 4.50 times). These STI-treated blastocysts took a significantly longer time for weak contraction and for re-expansion after weak contraction (10.2 and 87.6 min), compared with non-treated blastocysts (7.8 and 58.2 min). It was also confirmed that the activity of trypsin-like proteinase was similar in STI-treated and non-treated blastocysts, and that a small hole was formed in the zona pellucida at the start of hatching in STI-treated blastocysts, as seen in non-treated blastocysts. Never-theless, the rate of blastocysts with a slit resulting from enlargement of the small hole in the zona pellucida was 37.5% in STI-treated blastocysts, which was significantly lower than the 86.7% in non-treated blastocysts. These results suggest that weak contraction and re-expansion after weak contraction of blastocysts play an important role in hatching by slitting the region adjacent to the small hole in the zona pellucida.