Systolic blood pressure was measured in males of 8 inbred strains and 1 outbred stock of rats 5 times over a period of 20 min on 5 consecutive days. The strain means ranged between 107·9 mmHg and 149·3 mmHg. The estimated variance between strains (V = 248·7 mmHg) was about 5 times higher than the variance within strains (V = 54·3 mmHg). The intraindividual variance within strains was relatively constant (V = 24·0-37·6 mmHg), while the interindividual variance varied to a great extent (V = 4·5-44·5 mmHg) from strain to strain. The outbred stock showed values of blood pressure and components of variance similar to those of a single inbred strain. Thus, by investigation of a battery of 8 inbred strains in a multifactorial experimental design a greater phenotypic variability due to genetic strain differences is achieved than by measurements in a single outbred stock.