Seven bitches of several breeds were monitored during oestrus by vaginal smears and luteinizing hormone (LH), oestradiol and progesterone plasma assays. Each bitch was inseminated into the uterus with frozen spermatozoa from seven dogs of different breeds. Each female was inseminated daily with about 200 x 10(6) motile spermatozoa. Six bitches delivered. The paternity of puppies (which indicated the fertilization time) was determined by morphological analysis. Two bitches were fertilized with one artificial insemination (AI) only and four bitches with two successive AIs. Fertilization occurred 84-180 h after the LH peak and 1-3 days before the first day of metoestrus. The gestation length, from fertilization to birth, was 58-63 days or 56-60 days after the first day of metoestrus.