The freezing conditions for preparation of frozen canine semen by the plunging method were investigated with regard to the period of sensitization in liquid nitrogen (LN2
) vapor and the height from LN2
, and the semen qualities after thawing were compared with those of canine semen prepared by the simple freezer method previously reported by us. In the plunging method, 9 semen straws were prepared under the same conditions, horizontally kept at 5, 7, and 10 cm above the LN2
surface in a styrene foam box for 5, 10, and 15 min, and then plunged into LN2
. The semen qualities immediately after thawing were high in the 7 cm/10 min (cooling rate: -4 to -22°C/min) and 10 cm/15 min groups (cooling rate: -6 to -10°C/min). On comparison of frozen semen prepared by the plunging method (7 cm/10 min) with frozen semen prepared by the simple freezer method, sperm motility and viability were significantly higher for the frozen semen prepared by the plunging method. The cooling rate in freezing was higher for the simple freezer method (cooling rate: -6 to -50.9°C/min) than the plunging method. Based on these findings, horizontal placement of canine semen straws above LN2
to reduce the temperature at a slow cooling rate of about -10°C/min, followed by plunging into LN2
after sensitization for 10-15 min, provides good semen qualities after thawing.