Diluents based on the zwitterion buffers Tes, Hepes and Pipes titrated to pH 7.0 with NaOH or Tris were compared with Tris-citrate diluents by assessment of post-thawing motility and acrosome integrity of frozen ram spermatozoa. Varying buffer osmolalities between 300 and 360 mosmol had no effect but there was a decrease (P < 0.001) in motility between 360 and 420 mosmol. There was a quadratic effect (P < 0.001) on motility of increasing glucose concentration in the diluent (30, 85, 140, 195 and 250 mM) with a maximum at 85 mM. At this sugar concentration, motility was higher (P < 0.001) for diluents containing glucose and fructose than lactose, sucrose or trehalose. Inclusion of egg yolk (optimum at 13.5% v/v; P < 0.05), centrifugation of the diluents (P < 0.01), use of low dilution rates (3 or 6-fold; P < 0.05) and freezing in pellets rather than minitubes or straws (P < 0.001) improved both the motility and acrosome integrity of spermatozoa in the zwitterion diluents. It is concluded that zwitterion buffers are superior to Tris-citrate in ram semen freezing diluents.