Split ejaculates from four boars were frozen with a programmable freezing machine, in mini-(0.25 ml) and maxi-(5 ml) plastic straws with an extender at either acidic (6.3) or alkaline (7.4) pH. Glycerol (3%) was used as cryoprotectant. The freezing of the semen was monitored by way of thermocouples placed in the straws. Post-thaw motility and acrosome integrity were evaluated; the latter using phase contrast microscopy, eosin-nigrosin stain and electron microscopy. Post-thaw sperm motility was significantly higher when semen was frozen in mini-straws than in maxi-straws. For the mini-straws, the motility was better when semen was exposed to an acidic environment during freezing, but this beneficial effect of the low extracellular p H was not evident when maxi-straws were thawed. The motility of the spermatozoa diminished significantly during the thermoresistance test (0 h and 2 h time) at 37°C in a similar way for both straws and extracellular pH's. The freezing procedure, no matter the extracellular pH, did not cause major acrosomal damages, but significantly more normal apical ridges were present in the mini-straws than in the maxi-straws. This in vitro evaluation indicated that the freezing method employed was better for mini-than for maxi-straws since the freezing of the 5 ml volumes was not homogeneous, due to the large section area between the surface and the core of the straw.hygienic (HAMMITT and MARTIN, 1989). For practical reasons, a larger frozen volume (i. e.