Rats develop pulmonary hypertension over a 2-wk period of continuous ingestion of monocrotaline dissolved in drinking water (20 mg/l). The relationship between monocrotaline concentration and duration of exposure was investigated by giving male rats (initial body weight 100 g) monocrotaline in drinking water (5, 10, 20, 40, or 60 mg/l) for 0, 1, 2, 4, 6, 10, or 20 d. Rats were killed 20 d after initiating treatment, and increased lung and right ventricular to body weight ratios were measured as indices of pulmonary hypertension. The accumulative dose of monocrotaline delivered over a 10-d period using a drinking water concentration of 10 mg/l (18 mg/kg) produced the same degree of right ventricular hypertrophy and lung weight increases as the doses ingested over a 4-d period by rats consuming 20 or 40 mg/l monocrotaline water (14 and 29 mg/kg). Phenobarbital pretreatment did not substantially alter the time course of toxicity induced with 20 mg/l monocrotaline water. Ingestion of 60 mg/l monocrotaline water for 1 d (11 mg/kg) resulted in right ventricular hypertrophy at 20 d. Since accumulative doses of less than 11 mg/kg did not produce toxicity and all doses greater than 14 mg/kg did, this range may be considered a threshold for inducing toxicity. However, organ weight increases following threshold exposures reversed over a 4-wk period. Increases in the wall thickness of pulmonary arteries correlated with the development of right ventricular hypertrophy. Pulmonary inflammation was not an early response to monocrotaline administration, since there was no change in the proportion of cell types recovered in lung lavage fluid during the first 6 d of monocrotaline treatment.