The aim of the study was to assess the effect of subacute treatment with a low dose of atrazine (1,3,5-triazine-2,4-diamine, 6-chloro-N-ethyl-N'-(1-methyl-ethyl), an s-triazine herbicide, on endocrine oestrus regulation in gilts. A group of nine gilts (F1 generation of Swedish Landrace x Large Yorkshire) were treated with 1 mg atrazine/kg body mass daily, mixed to the feed for 19 days before the onset of expected oestrus. Blood samples were obtained by cranial vena cava puncture three times daily at 3-h intervals on five post-treatment days, i.e. before and during oestrus. The serum concentration of oestradiol-17 beta (E2) was determined by the fluoroimmunochemical method. On Day -2 before the onset of expected oestrus, a significantly lower (P < 0.001) E2 concentration was measured in the serum of treated gilts (31.25 +/- 1.95 and 39.32 +/- 1.38 pg/mL) than in the control pigs (51.43 +/- 1.29 and 68.59 +/- 2.99 pg/mL). In contrast, the E2 concentration measured in the serum of treated animals was significantly higher (P < 0.001) on the day of the expected onset of oestrus and on the subsequent two days (35.43 +/- 1.85, 53.92 +/- 1.98 and 60.32 +/- 2.35 pg/mL, respectively) than in the control animals (13.52 +/- 1.79, 21.53 +/- 1.35 and 20.05 +/- 1.46 pg/mL, respectively). Insufficient serum E2 concentration of the treated gilts resulted in a failure of expected oestrus, as indicated also by the state of dioestrus demonstrated by histopathological examination of the uterus.