There have been few reports of artificial insemination using fresh feline semen because of difficulties in collecting semen. It has been shown previously that the number of spermatozoa required for fertilization of cats by intravaginal insemination and unilateral intrauterine horn insemination are 8.0 x 10(7) and 8.0 x 10(6), respectively. There have been no reports of intratubal insemination of cats. Therefore, a study was designed to determine the number of spermatozoa necessary to fertilize cats by intratubal insemination with fresh semen. Four male and 25 mixed breed female cats were used. Semen was collected using an artificial vagina. Insemination was performed by laparotomy 15-30 h after administration of 100 iu hCG on day 2 or 3 of spontaneous oestrus. Cats in which ovulation had not occurred at insemination received an additional 100 iu hCG after surgery. In intratubal insemination, four groups consisting of three, seven, eight and seven cats received 5.0 x 10(3), 5.0 x 10(5), 2.0 x 10(6) and 4.0 x 10(6) viable spermatozoa, respectively, into both uterine tubes at approximately 2 cm from the fimbriae tubae. The volume inseminated was 10-20 mul. The semen volume was adjusted with egg yolk-Tris-fructose citrate. As a result of artificial insemination, none of the cats that received 5.0 x 103 or 5.0 X 105 spermatozoa were fertilized. However, fertilization was successful in two of eight animals (25.0%) that received 2.0 x 10(6) spermatozoa and in three of seven animals (42.9%) that received 4.0 x 10(6) spermatozoa. The number of kits was 1-4 (mean +/- se: 2.4 x 0.6). These results indicate that although large numbers of spermatozoa were inseminated into the uterine tubes, the conception rate was low. The low rate of conception may have been due to problems in inducing capacitation of spermatozoa in the uterine tubes.