Semen collection from three boars was performed twice a week, and doses (100 ml each) of semen diluted with Beltsville Thaw Solution (BTS) extender were prepared containing 1 × 109, 3 × 109 and 5 × 109 spermatozoa. Diluted semen was then stored at 17°C for a maximum of 120 h. Percentage of motile spermatozoa (PMS) and type of spermatozoa motility (TSM) were determined by microscopic examinations performed at 24-h intervals, from zero time (beginning of storage at 17°C) up to 120 h of storage, and a sperm motility index (SMI) calculated. Sows (no. = 360) were divided into nine experimental groups (40 sows per group) and a combination of number of spermatozoa per dose and storage time was used for artificial insemination in each group. All the animals were inseminated twice at 12 and 36 h after detection of standing oestrus by a teaser boar. Return and farrowing data including litter size were recorded. PMS, TSM and SMI decreased significantly (P < 0·01) from 48 h up to 120 h of storage, this being more marked in semen doses of 5 × 109 spermatozoa. When semen stored for up to 24 h was used, no significant differences (P > 0·05) regarding return rates and farrowing rates were observed among the three groups inseminated with semen containing different numbers of spermatozoa per dose. Inseminations with semen doses of 1 × 109 and 3 × 109 spermatozoa stored for 48 to 72 h have significantly lower return rates (P < 0·05) and significantly higher farrowing rates (P < 0·05) than with doses of 5 × 109 spermatozoa. Inseminations with semen stored for more than 72 h gave relatively higher return rates and lower farrowing rates. Semen storage time and number of spermatozoa per dose appeared to reduce litter size only where semen doses of 5 X109 spermatozoa were inseminated after storage for more than 48 h.