Subsecond fast low‐angle shot (FLASH) magnetic resonance imaging(MRI) allows single shot studies of the human heart within measuring times of about 100–300 ms depending on the data matrix. In contrast to conventional FLASH MRI subsecond applications acquire data d u r i n g the approach to steady state. A detailed analysis of the saturation behavior of the signal is given for the ideal case of a rectangular slice profile. In a second step, realistic slice profiles assuming Gaussian‐shaped excitation pulses were taken into account by means of a numerical solution of the Bloch equations. It turns out that the signal strength and the resulting image intensity is considerably higher than may be expected from steady‐state considerations. Correspondingly optimized flip angles depend on the number of phase‐encoding steps. Assuming long T 1relaxation times as, for example, encountered in muscle and brain tissue and repetition times of 5 ms or less, optimum flip angles are 12°–16°. The use of even higher flip angles (≥20°) causes heavily distorted slice profiles and a dynamic increase of the effective slice thickness. Flip angles of the order of the Ernst angle (6°) correspond to steady‐state conditions and lead to considerable signal losses. The theoretical results are confirmed by subsecond FLASH MRI studies of the human heart using a 2.0 T whole‐body system (Siemens Magnetom).