The sperm count required were investigated to obtain a conception rate of 80% by unilateral intrauterine insemination (UIUI) of fresh semen in cats. The conception rates obtained by insemination before and after ovulation were also examined. Thirty-six female cats aged 1-7 years were used in the experiments, and the number of experimental cases was 44. Seven male cats aged 2-12 years from which semen could be collected by the artificial vagina method were used. In artificial insemination, 100 iu x 2 or 250 iu of hCG was administered on days 2-4 of estrus, and sperm were introduced into the uterine horn with a greater number of ovulations (or mature follicles) 15, 20 and 30 hr after hCG administration by laparotomy. The inseminated sperm counts were 2 x 10(6) (Exp. 1), 4 x 10(6) (Exp. 2), and 8 x 10(6) (Exp. 3). As a result, ovulation was induced in 42 of 44 cases (induction rate: 95.5%) regardless of the dosage of hCG. Conception was obtained by UIUI in two of 16 animals (conception rate: 12.5%) in the Exp. I, five of 16 animals (31.3%) in Exp. 2, and eight of 10 animals (80.0%) in Exp. 3. Regarding the relationship between the ovulation state at insemination and conception, the conception rate obtained by insemination before ovulation was clearly higher than that obtained by insemination after ovulation (p<0.05). Regarding the number of kits compared to the number of ovulations on the inseminated side, the percentages of cases in which the number of kits exceeded the number of ovulations on the inseminated side were similar in all groups inseminated with a different number of sperm. It is therefore necessary to investigate conception rates obtained by bilateral insemination to increase the fertility rate. Based on the above findings, it was shown that the sperm count required for fertilization by UIUI is 8 x 10(6).