The contractile activity, the threshold for calcium-induced calcium release in fractions of sarcoplasmic reticulum and the potassium concentration were determined in preparations of semimembranosus muscle from normal quarter horses and quarter horses with hyperkalaemic periodic paralysis before and after they were treated with phenytoin. Before the treatment there was no difference in caffeine contracture or electrically elicited twitch response between the two groups. For one week after the treatment, the time to peak tension of caffeine contractures was significantly (P < 0.005) reduced in the horses with hyperkalaemic periodic paralysis but unchanged in the normal horses. The variance but not the mean values for the threshold for Ca(2+)-induced Ca2+ release from the sarcoplasmic reticulum was greater for the horses with hyperkalaemic period paralysis before but not after the treatment with phenytoin.