Cumulus-enclosed pig oocytes were matured in vitro, freed from cumulus cells, and inseminated with frozen-thawed ejaculated spermatozoa in a chemically defined protein-free medium containing 37.0 mmol NaHCO3 l-1 and 5 mmol caffeine l-1. When the medium was supplemented with 1 mg polyvinylalcohol (PVA) ml-1, more penetrated oocytes were observed 14 h after insemination with 7-12 x 10(6) cells ml-1 than with 4-5 x 10(6) cells ml-1 and the incidence of polyspermy reflected the sperm concentration used. Varying the NaHCO3 concentration but maintaining the sperm concentration at 8 x 10(6) cells ml-1 resulted in significantly more oocytes being penetrated in media containing 45.83-50.25 than 37.0-41.42 mmol NaHCO3 l-1; there were no significant differences in the incidence of either male pronuclear formation or polyspermy. In medium containing 45.83 mmol NaHCO3 l-1, the inclusion of PVA at 0-5 mg ml-1 had no effect on proportions of penetrated oocytes, male pronuclear formation or polyspermy. However, when spermatozoa from three different boars were evaluated, the penetration and male pronuclear formation rates were highly variable, unlike the incidence of polyspermy. Penetration of cumulus-free oocytes was first detected at 6 h. When spermatozoa were incubated for 6 h in the absence of oocytes, motility, but not vitality, decreased whether or not PVA was included in the medium. Chlortetracycline (CTC) fluorescence analysis of the capacitation state indicated a rapid decline in the proportion of live uncapacitated, acrosome-intact cells and a rapid rise in the proportion of live capacitated, acrosome-reacted cells during the first hour.(ABSTRACT TRUNCATED AT 250 WORDS)