Sperm reactivation as a function of extracellular osmolality. Before assay, sperm were immobilized by transfer from seminal plasma into 360 mmol/kg TES-buffered potassium chloride, pH 7.4, containing 5 mM 1,2-bis-(o-aminophenoxy)ethane-N,N,N’,N’-tetraacetic acid (BAPTA). This was achieved by centrifugation through 12% (wt/vol) Accudenz. Washed sperm (n = 5 replicate pools) had a concentration of 5.5 ± 0.20 × 109 sperm/mL (mean ± SD). Washed sperm were held briefly at 20°C before assay. Sperm were reactivated at 41°C with 2 mM Ca2+ in TES-buffered sodium chloride, pH 7.4, with varying osmolality. Each open circle is a mean ± SD. The solid line denotes the logistic function y(x) = /[1 + e0.098 (339– x)], where 75% recovery denotes an asymptote. This experiment demonstrated that the sperm mobility assay could be used to define how sperm recover from an imposed, non-physiological condition.