Abstract
A defined medium (CCM) is described in which washed ejaculated canine spermatozoa can be induced to undergo capacitation and the acrosome reaction, and to penetrate corona‐free eggs in vitro. The composition of the medium is similar to other Krebs‐Ringer's bicarbonate buffered media except for the absence of magnesium and adjustments in the concentration of NaHCO~3~, glucose, and albumin. The concentration of NaCI is adjusted to maintain the osmolality at approximately 300 mOsm, and the pH is 7.8 under 5% CO~2~ in air. This pH was found more favorable for the occurrence of the acrosome reaction than the lower pH (7.4) of media with more usual bicarbonate concentrations. Calcium ions are essential not only for the occurrence of the acrosome reaction of canine spermatozoa, but also for their motility. Potassium ions are apparently necessary for the process of sperm‐egg fusion following zona penetration. When CCM was compared with two other media, it allowed the best combination of good rates for acrosome reaction (46%), motility (83%), and zona penetration (71%). Sperm‐egg fusion also occurred. The requirements for capacitation of canine spermatozoa are compared with those of some other species.