Apparatus and methods for injecting biological agents into tissue. Devices are provided having elongate shafts and distal injection heads for driving needles into tissue and injecting medical agents into the tissue through the needles. A longitudinal force directed along the shaft can be translated to a needle driving force. Some devices provide controllably variable needle penetration depth. Devices include mechanical needle drivers utilizing four link pantographs, rack and pinions, and drive yokes for driving a first needle bearing body toward a second tissue contacting body. Other devices include inflatable members for driving and retracting needles. Still other devices include magnets for biasing the needles in extended and/or retracted positions. The invention includes minimally invasive methods for epicardially injecting cardiocyte precursor cells into infarct myocardial tissue.